The October afternoon before he is to fight the American, Thub Hong-Mo sits cross-legged on the floor of his Bangkok prison cell, watching soap operas on an old Chinese TV set. It is a rare moment of respite for the 30-year-old, who has spent the past two months – seven days a week, six hours a day – working out relentlessly in the prison boxing gym. His lanky frame is nothing but raw muscle and sinew; his shins are knotted with scar tissue. Across his chest is a vibrant tapestry of tattoos: shimmering fish scales, a pair of demons, and a leering dragon. The dragon, he explains through a translator, is a recent addition, commemorating what he calls his "long journey" in one of the most notorious prisons in Thailand.

Four years ago, Hong-Mo was found guilty of stabbing a man to death in a Bangkok nightclub. He is now serving 35 years at Klong Prem prison. Most inmates here are career criminals, with sentences of at least a decade – rapists, gangsters, murderers. It's where you go, as one lifer puts it, "when they've decided that they never want to see you again."

Hong-Mo had no reason to think he'd leave this place until he was well into middle age. Then last July, prison authorities approached him with a proposal: Fight against a Western professional in a martial-arts match, to be held in the yard at Klong Prem, and they promised to put 5,000 baht, or $150, in his commissary account. If he acquits himself admirably, they suggested, they may even cut a few years off his sentence.

The discipline: Muay Thai, a balletic but bloody commercial sport that blends the basics of boxing – jab, cross, uppercut – with kicks, knees, elbows, and clinches. The event is being organized and recorded by a brash young Estonian promoter with a Barnum-ish flair for spectacle, who hopes to cash in on the growing international market for fight DVDs. Billed as the Battle for Freedom, it is to consist of seven matches, each pitting a Klong Prem inmate against a Westerner. (That the competitors will come from Europe and the U.S. – where Ultimate Fighting Championship [UFC] pros like Anderson Silva have popularized the sport and most major cities have muay Thai gyms – will only add to the spectacle.) Although the crowd will consist solely of fellow inmates, the promoter hopes to eventually reach a much larger audience through DVD sales.

Like many inmates, Hong-Mo grew up practicing muay Thai; for a time, he even fought semiprofessionally. He eagerly agreed to participate, as did one of his cell mates, a drug trafficker named Wuttipong Korsanthiet, 27, who goes by the nickname Moo, or "pig." "In prison you can lose something of yourself if you're not careful," Hong-Mo says. "For me, the fight is about proving to myself that I'm still a person. That I still have pride."

Hong-Mo is preparing to face Mark Sayer, a 32-year-old brawler from the suburbs of Houston who'll be fighting for the first time in Thailand – the epicenter of muay Thai. Moo is matched against Stephen Meleady, an unranked Dubliner with a professional record of 43 wins and 31 losses.

"Tomorrow I will win," Moo says. "It's in my blood." As if to prove it, he drops into his fighting stance, executes a flurry of rabbit punches, and then spins his body and knocks an elbow into his imaginary opponent's face.

Still seated on the cell floor, Hong-Mo shakes his head. "I will let the victory speak for me," he says.From the outside, the whole thing seems a bit outlandish. But the Thai penal system has a long history of organizing sporting events, from soccer and basketball tournaments to weightlifting competitions, and – in a process that is as seemingly arbitrary as it is opaque – of occasionally dishing out sentence reductions to the athletes viewed as bringing honor to their country. In the 1980s, when Thai authorities, as part of an effort to modernize the penal system, introduced a program called Sports Behind Bars, muay Thai – "Thai boxing" – was one of the first activities offered. Since then, a handful of prisoners have managed to parlay their skill into an early release. In 2007, the imprisoned drug dealer Siriporn Taweesuk beat a Japanese boxer for the World Boxing Council light-flyweight title in a match held at Klong Prem. Not long afterward, she was released, having achieved, in the words of one Thai official, "glory for Thailand." And that same year, Amnat Ruenroeng, a muay Thai veteran and convicted robber serving 15 years at Bangkok's Thonburi prison, was pardoned after winning a national title in boxing. Ruenroeng later went on to compete for Thailand's boxing squad in the Beijing Olympics.

In 2012, Kirill Sokur, a 35-year-old Estonian émigré and fight promoter, helped devise a new breed of behind-bars event: one that would match up, for the first time, Thai inmates and Western pros. He called his event Prison Fight and came up with the suitably catchy Battle for Freedom slogan. Sokur offered prison officials a deal: He'd provide the ring and the Western fighters and drum up attention from local newspapers and TV stations. The matches would be dubbed "charity events" – a nod to the fact that rehabilitated prisoners could earn their freedom through battle – which would make the prison brass look good. In return, Sokur would film the fights with an eye toward eventually selling DVDs or perhaps producing some kind of reality show. Working with funding from an undisclosed source, Sokur has poured thousands of dollars into his venture in the hopes of finding a ready audience: Buoyed by the UFC's worldwide success, the international market for fight videos has soared in recent years to become an annual multimillion-dollar industry.

"I thought, You could have a murderer on one side and a professional on the other," Sokur explains. "I knew it would be exciting for people. If a man is a killer outside the ring – if he's learned to kill – think about what might happen inside the ring!"

The first three Prison Fights were held in early 2013 at Klong Pai prison, a medium-security complex in the Nakhon Ratchasima province, 100 miles north of Bangkok. Although two-time boxing champion Oh Singwancha won his freedom at a Prison Fight muay Thai bout, the events did not generate the kind of media coverage Sokur had hoped for. He decided to aim bigger: a fight at Klong Prem, in the capital, with several camera crews on hand, as well as a punk rock band and at least one well-known Thai socialite. "This will be our breakthrough," Sokur says the day before the fight. "The biggest show, the biggest stage."For Mark Sayer, the American scheduled to fight Hong-Mo, the journey to Klong Prem began almost two decades ago, in the bedroom of his Texas home. It was there that Sayer stumbled across an advertisement in the back pages of Black Belt magazine for a book called Thai Boxing Dynamite: The Explosive Art of Muay Thai. Sayer, then a freshman in high school, bought the book and devoured it in a day.

He had tried boxing and tae kwon do, but now he made up his mind that his future lay in muay Thai. "I was drawn to the grittiness of it," says Sayer, who has the buzzed hair and unyielding gaze of a soldier. "The full-contact nature. And the fact that there was this whole culture surrounding the sport."

Muay Thai is known as the "science of eight limbs," a reference to the legal contact points: feet, knees, elbows, fists. As in traditional boxing, the action takes place in a square canvas ring, bounded by extra-strong rope, which the wiliest fighters learn to use to their advantage, drawing their opponents back into it as if it were a spiderweb or bouncing off it to launch a counterattack. Eye-gouging, head-butting, and crotch shots aren't allowed. Just about everything else is.

With the help of his father, a former Navy sailor and an amateur boxer, Sayer found a local gym run by a French émigré who taught muay Thai. Sayer was the youngest member there by several years. Most of the other guys, he recalls, were "bar fighters and rough dudes"; one of them once showed up with a prison tracking device on his ankle.

His first fight took place at a Thai temple in Houston. His opponent had 10 years on him, but Sayer soundly beat him. Afterward, bruised and bleeding but exhilarated, he was pulled aside by the referee. "Someday," said the ref, who owned a nearby Thai restaurant, "when you're 18, you'll come to Thailand and then you will really fight."

It took a little longer than that. After high school, Sayer spent five years at the United States Merchant Marine Academy in New York. He graduated in 2006 and spent the next seven years with various companies crisscrossing the globe: Brazil, Kuala Lumpur, Singapore, Ghana. During his off-weeks, he lifted weights and trained at gyms. He was getting older, but he was still strong and his reflexes hadn't dulled. Along the way, he fought in amateur muay Thai bouts and won most of them.

In 2013, after becoming certified to oversee a large commercial vessel, Sayer decided it was time to pause his career and pursue his dream. Last June, he hopped a flight to Bangkok, the muay Thai capital of the world, in order to fight as a professional. "I'm 32," he says. "I figure that, for a little while, my career can wait. For me, there's a short window of opportunity left to do muay Thai. This is about proving something to myself."

Thailand is full of gyms that cater to Western journeymen, but Sayer had his heart set on the well-respected Sitsongpeenong boxing gym, in the eastern suburbs of Bangkok. Most of the guys at Sitsongpeenong are Thai, but there are some farang, or foreigners, too – Westerners determined to make a name in the sport. In mid-September, his trainer relayed the good news: Sayer had been offered a fight – his first in Thailand. The purse was 5,000 baht, the same amount inmates were offered.

It was only when Sayer showed up at the prison for the press conference, a week before the fight, that he realized this was no ordinary match. "I remember following these prison guards out into the main yard and just trying to keep my game face on," Sayer recalls. By the time he was introduced to Hong-Mo, Sayer had decided that no matter how ludicrous the whole scenario was, it was too late to back out.

Sayer didn't ask what crime his opponent had committed. "When you fight someone, even if you don't actually know him, a type of bond is formed," he says. "For all I know, this guy could be a rapist. He could be using this fight to get out of prison. Maybe he's a gang leader. Maybe he's pulling strings from the inside. It's better not to know."

That night, Sayer fired up his laptop and posted a message to his Facebook wall. "We've all got our definitions of success," he wrote. "My friends back home are having kids, making money. Me? I'm participating in live blood sport in a Bangkok prison."Thub Hong-Mo grew up more than 9,000 miles from Mark Sayer, in one of the low-slung, impoverished suburbs that ring Bangkok. Like many poor boys in Thailand, he found solace in the sport of muay Thai. He was an explosive fighter, with strong hands and a stronger clinch, and he cultivated a trademark leer, which he wore even when he was being knocked around the ring. He found that it unnerved his opponents.

By the time he was 18, he'd racked up a decent record. He debated pursuing a pro career but instead accepted an offer to work as a debt collector for a gang in the Nana Plaza section of Bangkok, a hive of brothels and go-go clubs with the densest concentration of sex workers in the world. One evening in 2009, Hong-Mo was out with a few friends at a Bangkok nightclub. A scuffle turned into a melee, and Hong-Mo pulled a knife with a six-inch blade from his jacket pocket. "It was self-defense," he says, running his hand through his high pompadour. "I knew that if I didn't do it, he would do it to me."

The judge didn't buy that argument, and a few weeks after the stabbing, Hong-Mo reported to Klong Prem. He spent his first few months adapting to the indignities of prison life. He shares a 5-by-10-foot cell with four other inmates. There are no beds. They sleep on the floor, on blue foam mats. Space is so tight that Hong-Mo can feel their breath on his neck as he lies awake at night, listening to the cockroaches cascade down the walls in platoons. Every morning at 6 am, the cell door swings open; every afternoon at five it rolls shut.

The Prison Fight offer came as a relief. "It gave me something to look forward to, to put my mind on," Hong-Mo says.

Under the tutelage of Nikon Jangthinpha, a retired gangster imprisoned for triple homicide, Hong-Mo spent six hours a day in the prison's makeshift gym, which is fitted out with an old canvas ring and two leather heavy bags. In the mornings, he jumped rope, jogged, and shadowboxed; in the afternoons, he hit the bags and sparred with other inmates.

"I watched the transformation myself," says Jangthinpha. "Maybe at first he loses his breath, he has a little extra flesh around his stomach. Now he is ready, he is focused, he is strong."

Hong-Mo pauses when asked what he thought about the possibility of a pardon: "I try not to think about that. I think only about the fight." He shrugs and looks skyward, as if it is out of his hands.Fight day dawns clammy and gray, with a low cloud cover that hangs over Klong Prem, sealing the humidity inside. Mark Sayer has a plan, and it is to not get reckless or overeager. "Of course," he allows, "if I see the kill, I'm going to go for it." When Hong-Mo enters the ring, his eyes remain fixed on Sayer. He looks hungry. He looks possessed.

The first round goes poorly for Sayer. Hong-Mo manages to tie him up in a clinch. Using his waist for torque, the Thai drops Sayer hard onto the canvas. While they're both down there, he grinds his knee into Sayer's solar plexus. It's a classic cheap shot – but the judges, from down at their table, aren't the wiser for it. Two minutes in, Hong-Mo connects with a kick to Sayer's left side. The bruise rises quickly, the shade of an overripe eggplant.

The bell rings, and Sayer slowly totters back to his corner for water and an ice compress. He knows that the fight is slipping away. "I need to find a way to win," he says. "That's all."

As soon as the second round commences, Hong-Mo seems to sense that something has changed in Sayer. Instead of peppering him with punches and kicks, Hong-Mo slides backward in stutter steps. The first good punch from Sayer is a jab, and it catches Hong-Mo on the side of the head. Next, a cross that slaps into his chin. A thin line of blood trickles down Hong-Mo's face toward his neck. Sayer's growing confidence is palpable – in the number of punches he throws, in the way he is wielding his weight to his advantage.

In the third round, with just a minute left in the fight, Hong-Mo drops his guard – this time not to tease Sayer but because he is clearly fatigued. Sayer pounces. His right hand sinks deep into Hong-Mo's cheek, smushing his face into a cartoon mask of surprise. Hong-Mo has enough instinct to clinch. Sayer steers him back into the ropes and, steadying his opponent with one hand, nails him in the face with the other.

The referee darts between the fighters, disentangling them, and the match is over. To Sayer's surprise, the decision is unanimous: All three judges score the fight in favor of Hong-Mo. Standing ringside, Sayer claims the ref snatched Hong-Mo from the jaws of ignominious defeat: Typically, fighters are separated only when all movement has stopped. "I was punching him," Sayer says. "I think that's movement."

Despite the loss, which he chalks up to "home field advantage," Sayer is gleeful. He believes that he gave out more punches than he took, and in his heart, that's enough to make him a winner.

Hong-Mo stands next to the referee, his nose swollen to the shape and hue of a cherry; a sheen of blood coats his teeth. He sways uneasily. "It was a big challenge," he admits. "But I fought like a warrior."

A prison guard arrives to march Hong-Mo back to his cellblock. A jubilant knot of fellow inmates awaits him there – he is the closest thing the prison has to a genuine celebrity. But the victory party is short-lived. That night, he is again slumped on the tiled floor of his tiny cell.

In past Prison Fights, news of sentence reductions has come quickly – Chalernpol Sawangsuk, an inmate competitor in the third event, was released shortly after his July victory over British professional muay Thai fighter Arran Burton. This time around, the process takes longer, due in part to the wave of violent anti-government protests that begins sweeping over Thailand in November. Hong-Mo's and Moo's sentence reviews are pushed back to April. In the meantime, the two prisoners must wait.

A few days after the fight, Mark Sayer sits in a sparsely decorated room he rents at the Sitsongpeenong gym. Against one wall is his sparring gear – shin guards, headgear – and on his bedside table, a book by Bill Bryson. Nearby is a grid of Post-its inscribed with various English phrases – "Could you please shut the door for me?" and "Speak more slowly, please" – and on the back, the Thai equivalents.

Despite his initial joy, he now finds the Klong Prem experience troubling. "I was enjoying the admiration of the crowd," Sayer says. "But I'm in this sea of convicts, who are patting me on the back and saying, 'Good job.' Some of them probably deserve to be where they are. And I was fighting someone who was fighting for his last shred of dignity. As a competitor, you kind of block that stuff out. But as a man, you have empathy."

In late December, Sayer is coming off his second fight in Thailand, at an arena in Pattaya. He's won by knockout – "Left hook to the jaw, overhand right to the back of the skull," he writes in an email – and he's in a contemplative mood. "I was thinking about the moment I passed my chief mate/captain's exams. I was like, 'OK, you accomplished this; now it's time to win a fight before you're too old....' I quit a six-figure job, gave up my awesome apartment, and now I've done exactly what I said I would do. It feels good." Now it is time to come home.