Originally Posted by NickC
Swim instead. Or use a kickboard if your strokes suck. Do a hundred laps a day, 3-5 days a week. Your knees will feel better and you will be fitter than anyone else you play.
Swimming is great because of its non-jarring nature.
The shoulders get a workout to better prepare them for serving - especially if swimming all the strokes, particularly the backstroke.
The only problem with swimming is that because the body "floats" and thus is essentially weightless, you have to swim pretty fast for a good cardiovascular workout that would directly benefit you for tennis.
Plus ... having an indoor pool gets you in from all that snow in Tuscon!