When you hit a tennis ball, your head should remain above your center of gravity and stay relatively still at all times. The more your head is over your center of gravity, the more balance you’ll have and the more efficiently you will track the ball and recover for your next shot. If your head moves too far from your center of gravity as you run to a shot or as you swing, your strokes will suffer.
Think about keeping your head over your belly button and try not to bob up and down when you run. One drill you can employ to help with this is the “folded towel drill.” Fold a towel and balance it on your head (your partner should do the same). If you lose the towel during in a rally, you lose the point.