For tennis, as much as I hate doing them myself, I'd recommend shuttle runs. Back and forth between the boundary lines over and over.
That, combined with ~30 minutes of mid-speed running every day should give you the agility and quickness as well as endurance for the sport.
Gah, I hurt just thinking of that.
Edit: Always remember that there is a serious difference between quickness and fitness. For most sports, including tennis, you have to train both.