I Am A 59 Year Old Male. I Have Been

I am a 59 year old male. I have been playing tennis all my life. Recently, in the past 6 months or so, my right knee hurts and is very stiff for days after I play. It takes about 2 weeks to feel better. When I go back to playing tennis again, my knee becomes sore again. It's difficult to (literally) put your finger on the location of the pain. When injured like that, if I don't move my knee for a while, it becomes very stiff, painful and difficult to move again, almost as though my body has created its own cast around the knee to immobilize it.