Take a good look at your hips. There’s a chance that they are not in balance: one edge may be higher than the other, or is more forward than the other. In worse cases, the hip can be both up and forward and completely out of balance. Tennis is a strenuous activity, working all the muscles of your lower and upper body. Unfortunately, most of the tennis players are one-side-only dominant, which creates more overuse of some muscle groups than others. The legs and hips are loaded with huge forces each time you start and stop, change the direction, and hit your strokes.