The more you bend your knees while hitting your shots, the better balance you will have. With a good low base, you will drive through the ball better, as it is much harder to open your hips too early and pull off the shot. Like a thin long stick that can be easily tilted over, if you stay tall and high, you can get easily off balance and the oncoming ball will play you instead of you attacking the ball. Stay low and compact and use your body to create a heavy ball. It takes a lot of strength and energy to be bending your legs deep for several hours on the tennis court. You have to be well prepared physically. The best preparation for this task is to do lunges.