No one likes being sidelined due to injury, so it's important to include exercises and stretches in your fitness plan that help prevent injuries. Heck, even if you don't exercise regularly, you should at least do these simple exercises which will help to strengthen, stretch, and mobilize the hips, calves, shins and ankles.
Do them while you're watching TV... do them while standing in the grocery line... do them, and be injury-free so you can play more tennis!