It's not just speed that's important, but the ability to decelerate and then rapidly accelerate in another direction. Your body must repeat this stop-and-go series thousands of times during a match, and it's the decelerating part where technique most often breaks down. It's also the part that's most demanding on your body.
So work on exercises that improve your ability to decelerate and change direction quickly, as well as improve balance, because regardless of playing surface or skill level, if your balance is poor, then you won't be able to move efficiently and with power.
Here's a drill to start:
Set up by placing six pieces of tape (saucer cones or tennis balls work, too) a few feet apart along the alley of the tennis court (the lines for singles and doubles), starting at the baseline and zigzagging up toward the net.
Begin by standing on your left leg in the corner of the court, and then jump off one foot explosively and diagonally so that you land on your right foot at the first marker in front of you and to the right, with your feet pointing forward. Land softly with your hips back and down and your chest up. (Watch this video for correct bounding technique.) Hold for 2-3 seconds, then continue bounding through the alley in this zigzag pattern.
As you feel more comfortable with this sequence, try sprinting three or four yards forward at any point during the drill. Tennis is, by nature, a random game, so mixing in sprints at various points during the drill will help prepare you for what you'll face in a match. Tip: If you're training with a partner, have him or her shout out when to sprint during the drill so that it's unexpected.
-- Nick Anthony