Hi @marc919
I hope you can get back to playing tennis, I know of people who did, and if your doctor encourages you, that is reassuring.
I'm guessing you have already done the more extreme stretching and bending exercises. The most helpful of these for me was using our stair steps several times a day to force my knee into a forward bend. Standing on the bottom, put the TKR foot two steps up and bracing your arms on the steps above, push forward into as deep a bend as you can stand, hold for 30 seconds, relaxing and sinking deeper forward as you hold. It's been 1 and 2 years since both my knees were done, but I still do this whenever I think of it.
We have a poorly designed house with a full flight of stairs up to the kitchen and living room, so I was forced to do a lot of stair climbing during recovery just for daily chores, which probably helped regain flexion.
Sounds like you are considering MUA to try to get added range, if your doctor approves, I would go for it!