Cycling used to help me when playing squash and stamina. I would cycle outdoors for about an hour 4 times a week and do 100-200 meter sprints before weight training, but after a warm up, 3 times a week. I'm not big into weights. I just do 3 - 4 major exercises. Bench Press - Squat - Rows and whatever that pull-down machine thing is called. I'm thinking of replacing the Bench Press with Pushups instead. Obviously, I will need a less intense schedule if playing tennis regularly. Maybe half the frequency.