Five days left of Insanity, and I'm with those who say do it either right before (if it's a lighter workout; for example I like to do one of the 'recovery day' workouts before tennis as a great warm up) or right after tennis. At first doubling up is very, very tough, but like others say, you get more used to it. KEY, though, is to get yourself loaded up with good nutrition, carbs, etc. I was flat baked until I realized how important nutrition was to keeping the energy level up. (not to mention helping with the results)...
Of course, Insanity (more cardio, leg based) is different than P90, which from what I understand is much more about muscle building, so that might be even more difficult to manage alongside tennis, so be careful. Good luck to you, though. I'm totally sold on Insanity and a regular hitting partner of mine did P90X during an offseason and lost a crapload of weight.