I've also got another story to back this up;
A former pro at my club who was once in the top 80 in the world and has a win against Brad Gilbert (when Gilbert was #4) told us that he once got a call from Martina Navratilova's people to arrange a practice with her. This was in the 80's, of course, when she was in her prime. When he showed up he was specifically instructed not to beat her in any practice set, which he ignored and promptly spanked her. He ended up not getting paid for the practice - stiffed by Martina!