Besides preparation/electrolyte deficiency, what's causing the spasms?
The body is like a race car. But there is no changing bodyparts. The open is played over two weeks and it's brutal on the body. His back seizes up. If it lets go, he'll play..if not he will walk through it and lose. It's just too much stress on the body.
Also mentally, after you get into that kind of rythm for Wawrinka, it's tough. You get angry and reckless.
I played a final of a tennis tournament around 2000, and sprained my ankle. The guy got angrier and angrier, even asked me if I want to quit.. but I hanged in there and I managed to sneak back in.. and I won. My pont is that Nadal will be very relaxed and motivated, while Wawrinka will get frustrated and lose his rythm. Just watch.