Well...Amelie Mauresmo recently.
And of the top of my head the only other one I can think of is Helen Wills Moody back in the 20's...but I am sure there are more.
It was after her two slams, and she never came back quite the same.
I have had one, but I'm not a pro.
Edited to add:
She won her slams in 2006.
She had her appendix removed in March of 2007, after winning the Proximus Games in Belgium (and that diamond encrusted racquet!). That was her last title, until yesterday!
One year Guillermo Vilas had an appendectomy during the French Open. he probably would have been favored to win the tournament if he could have played.
I remember he had a special removal procedure which avoided his abdominal muscles so that he could make it for the '86 Wimbledon. 3 memorable years-a QF, a championship '87 and then a sort of a grudge match with Becker in '88. I recall some expletives exchanged when Becker imitated a roll over the net.
The OP probably wants to know because Guillermo Garcia Lopez just defaulted in San Jose because of Appendicitis. I recently had an appendectomy due to Appendicitis. I am a Tennis Coach and I have not fully recovered 2 months later. It's an awful illness and the surgery requires at least 2 months of recover.
For Pat cash to have returned after 3 weeks and made Quarters of Wimbledon is insane.
i had my appendix out last november and only took about two weeks off from tennis. However it did take me a little to get back into it i felt very weak since all i had done was rest since the surgery.