Happy Joints, Happy Body
Your ability to move is dependent on the health of your joints and the extensiveness of your active mobility. In order to excel at any movement based activity(everything), your joints must meet the prerequisites first. If you want to play tennis, your knees, hips, and shoulders should be stable and mobile within the necessary ranges. Any limitations within the joint will compromise the movement pattern which will cause compensation and possibly lead to pain and/or a repetitive injury.
Before you take up a sporting activity or anything else requiring unaccustomed joint patterns, you should seek instruction from a trained professional and learn how to optimize joint function. When the joints work properly, the ability to move will follow.
My favorite training systems for improving joint health:
-Functional Range Conditioning
-Active Isolated Stretching
-Stretch to Win Method
The above list includes systems that emphasize active mobility, which is much more important than passive flexibility.