A player's rest and recovery always, and inescapably, follows his energy and activity.
In fact, this seems true in all endeavors in the human and animal worlds.
Indeed, this is the life cycle which underlies all existence.
In tennis and sports, top performance - and injury-free longevity - will likely depend on much more than practice and training.
It will depend, as the best professionals know, on intelligent and systematic recovery of body and mind between periods of activity.
Here's an interesting 28 page booklet from the USTA discussing recovery in tennis.
Well worth reading!
"Recovery in Tennis" (Adobe PDF)