Recover quickly.
Credit: Luis Acosta / AFP / Getty Images

In Williams' mind, a serve is only good if she wins the point. The fastest serve in the world is pointless if it is returned well. "As you follow through, you always recover, because even if it's your best serve, it may come back," she says.

After hurling your body at the ball, immediately hop into the "split-step" ready position: Your back is erect, your are knees bent, and you're balanced on the balls of your feet, which should be spread shoulder-length apart. Similar to rebounding your own shot in basketball or running out a ground ball to first base, hustling through the play is a matter of discipline.

"You always assume that it's gonna come back, because if you don't cover the next play, it's nobody's fault except yours," says Williams. "It's extremely unprofessional."