Before around the age of ten, cross training is usually pretty easy because kids have more time for sports. It seems to be common belief that playing lots of sports when young is good for a child and that is correct, but when it comes to training specifically for tennis at a later age, cross training selection becomes very important. Some modalities are difficult to access depending on where you are located or what your financial situation is. It is important for athletes to pick sports or practices that are going to be specifically helpful to them.

muscular integration
Any of the following practices would be beneficial in enhancing proprioception. Slow movements where the athlete can concentrate and feel the muscles work together. Well integrated muscles can enhance tennis mechanics as the kinetic chain will be more rapidly understood and implemented by the athlete!
- tai-chi
- feldenkrais method
- yoga
- rock-climbing

footwork
Can be taught on the tennis court, but it’s much more effective if the player naturally learns how to coordinate foot movement.
- dancing
- fencing
- boxing
- hackey sack

team sports
Good for cardio-vascular endurance and they stimulate decision making calculations as other team members must use their energy for one another in order to produce success. This also changes the experience of the athlete as losses are suffered with others and victories are enjoyed with others. I believe these are important development lessons.
- soccer
- football (recreational American football. ex. 4 on 4)
- lacrosse
- field or ice hockey
- touch rugby
- ultimate frisbee

balance development
We must remember that the tennis court is flat, we wear comfy shoes so our balance is only challenged by the movements we make. Balance should be overloaded for this reason. This can reduce the chance of ankle and knee injuries as these modalities recruit new stablizers.
- rollerblading
- surfing
- skate-boarding (wear proper crash equipment)
- wake-boarding
- snow-boarding

cardiovascular endurance
cycling promotes secretion of natural lubricants in joints and swimming is great for bio-mechanics as it requires the whole body to participate.
- cycling
- swimming