Every tennis player should have one of these http://www.powerballs.com/
I have had one for a couple of weeks and so far can't get above 10,000+ rpm. My problem seems to be the grip, as my hand just isn't big enough to maintain a good grip, once it's spinning quite fast to take it, to the next level. I can spin to 10,000 every time so I hope it's just a question of practise and building up strength in the forearm muscles and wrist.
I got the powerball because I have a history of tennis elbow, golfers elbow, and wrist trouble. The elbow problems are under control if not gone, but the wrist is a pain, literally. I started using a mouse with my left hand instead of the right and that helped for a while. However I now have a strange lump on one of the wrist tendons, which I hope will go away or go down. I've had ganglions before on my tennis hand and they have gone after a few months. They usually occurred when the racket was twisted violently from late contact with a hard volley, groundstroke or return of serve. I used to use a spring hand grip, and although that helps with the forearm strength the constant opening and closing of the hand seems to induce some sort of wrist RSI, so I have cut that down. Anyway I figured the powerball would help with the wrist, and in the few weeks I've had it my wrist has been much better.
I want to make my powerball do the jet engine scream like the video shown in the link when Akis gets it to 15000+. Wow!