There are four types of serves in tennis: a flat serve (no spin on the ball), a slice serve (brush the ball on the side driving through the ball towards the court), a topspin serve (brush the ball from low to high), and a kick serve (brush the ball from low to high on a diagonal). There are also different targets: the middle of the T, an opponent’s body, and the outer corner. 

Selecting the type of serve is based on many factors – the strong hand of the opponent, the score, the weather, and other factors – but whatever choice players make, Williams advises them to be certain about what they're doing before they reach the line. She prefers a flat serve into the body of her opponent, because it often results in weak returns.

"It's my best serve, I love it," she says. "Basically I'm handcuffing you. It goes straight into your body and it's so fast that you have no room. Either that or it's twisting into you. It slides into your body and you can't hit it back."

Still, any good player will adjust to even the hardest serve if you hit it twice. Williams says to mix it up and chamber your truly most powerful serves until you need them. "If you're up in the game, you'll serve maybe your least favorite. That way, when it's time to play the serious point, you can serve where you want, and your opponent's not saying, 'I know she's going there or he's going there.' "