Thanks for this. I had been hitting the gym before doing both upper and lower-body on weights, and maybe twice a week on the treadmill and elliptical. In retrospect, however, I would guess that what caused this injury was some combination of:
Lack of aerobic training with stretching.
Not stretching and warming up before the game (easy opponent).
Not stretching after doing weight exercises on my legs (presses, lifts).
Here's an interesting study from the journal of Sports Medicine stating that, while stretching may have no benefits for sports such as cycling or swimming, in sports that need what they call a "compliant" tendon (such as soccer, football and by extension tennis) require stretching to prevent injuries.
I've read other studies in which stretching may cause injuries, but this is in cases where the muscle is stretched beyond its normal range of motion: