Ho, boy. I am starting to see an unfortunate pattern as I try to transform myself into a singles player. I am so tired after three games of singles that I need to retire. This is a problem. I come out of the gate on fire in singles. I get to the ball early, set up, do something good. I run down everything. And then I crash. I am panting, dripping sweat. And we are nowhere close to finished. I start not moving well, and I start missing. There is something really wrong here. I am fit for my age. I can sprint, run up hills, run up hills backward. I take a weekly private tennis lesson that is grueling. I can run at a moderate pace for an hour. I do core work and upper body work. Two questions, then. 1. What can I do to work on my fitness for tennis, like right now. I need instant results here, folks. I could go to the track and do some intervals (like, full sprint 100 yards, slow jog 100 yards, repeat). Or I could just continue playing singles and try to get into shape that way. I do have a delicate knee, so I need to be efficient because there is a limited amount I can do without causing myself problems. Is it better to sprint on a track, or are hills better? 2. What might I be doing in singles that is causing me so much exhaustion? My opponents don't seem all that fit, yet I am always the one who needs a crash cart.