You can do aerobic exercise that incorporates tennis training if you do shadow swinging with a racket at the same time. When I don't have time to get to the courts, I do this in a small yard. I set my countdown timer and then do a pattern of split steps footwork and swings followed by rest periods. Of course, I do this barefoot.
But if you aren't squatting 2.5x your body weight you really shouldn't be playing tennis, it just isn't safe.