Combined, Roger Federer, Rafael Nadal, Novak Djokovic, and Andy Murray have won 36 of the last 39 men's grand slam singles titles. But the dominance of the so-called Big Four is coming to an end, a point that might well be hammered home at this year's US Open. Nadal isn't even playing (he withdrew on account of a wrist injury). Djokovic, who won Wimbledon, played poorly in his two US Open tune-ups ("It's more than obvious I'm not playing even close to what I'm supposed to play," he told reporters after losing in straight sets to Tommy Robredo in Cincinnati). Murray has not been the same player since he underwent back surgery late last season, and while Federer has had a great hard court season, there are instances now where his game inexplicably abandons him for long stretches.  

All of which means that the Open is setting up to be a huge opportunity for a handful of players outside the Big Four. Here are the top contenders not named Federer, Nadal, Djokovic, or Murray:

Stan Wawrinka: The 29-year-old Swiss won the Australian this year, becoming just the third non-member of the Iron Quartet to win a major since 2005. Wawrinka has had a pretty bad summer — losing in the first round of the French and also suffering early losses in Toronto and Cincinnati — and is looking to New York to put some spark back in his game. He reached the semifinals last year, losing in an epic five-set match to Djokovic (he endeared himself to New Yorkers when he was interviewed on-court immediately after the match and said Djokovic was "fucking tough."). Wawrinka is the one guy on this list who has proven that he can win a major; the Open gives him an opportunity to prove that it wasn't a fluke. 

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Grigor Dimitrov: The young Bulgarian, who recently cracked the top 10 for the first time, is often compared to Federer; his strokes are clearly modeled after Federer's, and he plays with the same mix of power and elegance. Of the younger guys coming to the fore, he easily has the most complete game. However, he sometimes seems brittle in pressure situations; his nerves betrayed him at several critical junctures in his semifinal against Djokovic at Wimbledon (at one point, he double-faulted three consecutive times, and he blew four set points that would have extended the match to a fifth set).  Ashe Stadium is a cauldron compared to Centre Court Wimbledon; it's a tough place to win a first major. But if Dimitrov can tame the yips, the Open could be his breakthrough moment.

Milos Raonic: The giant Canadian has had a great summer, reaching the semifinals at Wimbledon and Cincinnati and winning in Washington. While he fell in straight sets to Federer at both Wimbledon and Cincy, the arrows are pointing in the right direction. He's still dependent on his first serve, but if he catches a few breaks (i.e. Djokovic or Federer loses early), he might be able to break through.

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Jo-Wilfried Tsonga: The spirited Frenchman has shown that he can beat anyone when his legs, arm, and brain are working in unison. In Toronto, he took out Djokovic, Dimitrov, and Federer en route to the title. When Tsonga is on his game, there is no one more exciting to watch. What he has not shown to date is the ability to play to his full potential over the course of two weeks and seven straight matches, which is what you need to do if you are going to win a major. He's 29; it's going to have to happen soon if it is going to happen. One possible omen: if you start hearing that the courts in New York are playing faster than they have in recent years, keep an eye on Tsonga. The courts in Toronto were playing quicker, and he feasted. 

David Ferrer: Probably no player has suffered more from the dominance of the Big Four than Ferrer. The diminutive Spaniard has reached one grand slam final and five semifinals, and he has been thwarted each time by one of the Big Four. But despite his size disadvantage, his groundstrokes are as potent as anyone's, and as he showed in Cincinnati, his serve has gotten a lot better. Like Nadal, Ferrer grew up on clay, but as ESPN's Brad Gilbert noted during the Cincy event, hard courts have become arguably his best surface, which bodes well for the Open. At 32, he's running out of chances to win a major. If the draw opens up for him in New York, he could make a deep run.

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Tomas Berdych: The lanky Czech, who is seeded sixth at the Open, has had bursts of inspiration during his career; he took out Federer and Djokovic to reach the Wimbledon final in 2010 (he lost in the title match to Nadal). To date, his best showing at the Open was a semifinal run in 2012. Berdych is 6ft 5in, and with his concussive first serve and powerful forehand, he can cause trouble for anyone. This year's Open could be his best chance to add his name to tennis's list of one-hit wonders.

Nick Kyrgios: The dark horse pick. The 19-year-old Australian ambushed Nadal in the fourth round at Wimbledon. So pressure is clearly not an issue for the kid. His huge first serve and forehand, along with his effortless, almost lackadaisical style, calls to mind Pete Sampras when he was 19 — which was his age when he won his first Open (and first major title), in 1990.