When can you play tennis after total knee replacement?

I had a total hip replacement in January, 2007. I was an avid tennis player until I could not move at all on the court. That was in the fall of 2005. I attempted some alternative treatments for over a year. Although the pain decreased in my day to day life, I could not do anything athletic. After the surgery, I played Golf, walking the course in March, two months after the surgery. I started hitting tennis balls in June but could not move very well. I slowly improved my strength and flexibility over the next year. It probably took 1.5 to 2 years where I felt totally comfortable and confident. My "bad hip" became my good hip. It's been four years now and I am still playing singles tennis. My range and speed are decreased but much of that is due to age anyway (I am 63). But I still have fun and am competitive at the 3.5 level. Recently, some groin pain. That has come and gone. I am still playing but mindful. I do exercises and stretches to keep myself going. I do not do much jogging at all. I find my hip bothers my much more if I ran than play tennis. I do elliptical training which brings no pain at all.