I agree with cybelle. No one should play in serious pain, but any top athlete in any sport goes out there with aches and pain. Geez, even I do, at my pathetic amateur level of tennis. It might very well be why Serena is struggling, I'm sure it's true, but that's the nature of sport, if you're not feeling 100%, you might not win that day.
I think Serena will win and it will be a great mental victory.