Lame_Backhand: I'm in a similar scenario as you are. Keep in mind that strong leg muscles (particularly quads) help knees and swelling the most. Some exercises I do:
- Bike (stationary or regular) at least 3x per week. If I go for long durations (i.e. 1+ hour) I need some days for my legs to recover before playing tennis. I almost always bike for 15 minutes before tennis.
- Get some thigh weights (don't get weights that go below the knees) and do some straight leg lifts on the floor. Also, sit on the edge of a chair and write the alphabet in the air with your heel.
- Leg presses – one leg at a time. Best to use a machine that lets you lay flat on your back. Never let your legs bend more than 90 degrees.
- Step up – use a step and hand railing and work on very controlled step up focusing on your quads.
- Stretch – stretch hams, quads, calves and if possible stretch your IT band (IT band doesn’t really stretch but the attachments might).
When I’m diligent, I usually do ok playing tennis 2-3 times per week.