Hips are extremely important for a tennis player. Tight hips limit the range of motion and ease of movement on the court and can cause muscle strains and tendinitis. For example, if you need to get low down for a shot and if your hips are tight, you will bend in the waist rather then flexing your hips and knees and that will cause more strain on your low back. Your shots are not going to be as powerful, because you cannot engage your core the same way. It is very important to keep the hip muscles loose and functional to play good and injury free tennis. Often the hips are tight because they are pulling against tight groin muscles. Before you can effectively stretch your hip muscles, you have to loosen up the groin muscles, for example with the seated groin stretch, and the laying down frog stretch.