When has this concern with his back come on? Just today or has been carrying it this week....?
I haven't seen his first two matches, so I don't know, but early in the match he felt it a couple of times. he's played a fair amount of tennis, had a very long flight and jetlag to deal with (they said they all went from Munich to San Francisco nonstop
), so it wouldn't be a shock if he had a muscle niggle here and there.
i'm assuming it's nothing serious, but when he should've won probably 6-3 6-2, playing that much more certainly won't help that's all.
I don't think his biggest problem tonight was his back, I think it was his mood/attitude, but because of that, I hope his back doesn't become a bigger issue, that's all i'm trying to say.