The issue is somewhat fuzzy because the standard is general reasonableness. But you reasonably expect to get hit by a ball, when playing tennis (or, in my opinion, when you are around a tennis court). So you really can't complain about getting hit, even if it results in a terrible injury. The question to ask isn't whether an injury occured (that is question of damages and not of liability), but whether risk of the the contact was implicitedly assumed by agreeing to play tennis/be on the court.
It should be noted that you can't assume the risk for criminal behavior (e.g., http://sports.yahoo.com/blogs/tennis...6582--ten.html
You should also note, in the above examples, that being allowed to go to trial does not mean that the plaintiff won. Instead, it is merely a statement that the case is not laughable and would be allowed to trial. So getting hit after playing, when you are walking off the court may be a situation in which you did not reasonably agree to get hit by the ball. The likelihood, however, is that case will lose because everyone has been hit by a ball when walkining on/off the court.
I can certainly think of examples of liability from getting hit by a tennis ball. I cringe to recall that years ago, I played tennis dodge ball at a kids tennis camp (trying to hit other players with the ball)- I can certainly imagine a situation in which the camp would be liable if one of the players was injured because it was stupid.