Wait for the ball.
How you want to throw the ball depends on your height. Taller players, like Williams, can reach higher and should take advantage of that with a high toss. The right measurement for the toss is the point at which your arm is fully extended and you are connecting with the ball when it has just barely begun to descend.
It is a common misconception that the best place to hit the ball is at its peak height. "If you catch it on the way up," Williams warns, "you're just going to throw your timing off."
Williams recommends making contact after the ball has started heading down, "because as you are going up, you've already bent your knees, so you're going to get up a little bit." In order to find a perfect spot, Williams suggests watching pros who have a body type similar to your own.