^Kieran, I only knew the Spanish word that Rafa used for it, "isquio," and I thought it was groin, but I looked it up and it is the hamstring. Minor detail. Anyway, that one could have been starting out with so much running from the get-go, as you can imagine between the two of them. Rafa usually comes in very warmed up, so it's surprising, but, as you suggest, TM, it might have been nerves that tightened him, as it could have been last night. In 2010 v. Murray, he said he felt something painful, but a new pain he didn't recognize in the knee, and he was scared about it, as he is understandably protective of his knees. He wasn't going to win that match, and he had a free pass owing from Murray, (having stayed in their 2009 Rotterdam final when he couldn't move in the 3rd due to his knees,) so retiring seemed the wiser course.
Some folks were pretty rude last night about Rafa and the MTO, and his injuries as a form of excuse, in general. (That's not pointed at you, TM, or this thread.) Nadal practices and plays very hard, which both makes him the player he is, and makes his body break down, sometimes. People who aren't looking for reasons to dislike him further would see that he's too competitive to quit or back away from a challenge.
Also, I was arguing even with my fellow Rafa fans a bit, and I don't think I made my point very clear. I absolutely recognize that Nadal was hurt, and compromised, which I believe every one can see is a fact. What I didn't want to do was engage in conjecture, i.e., that Rafa would have won the match if he hadn't hurt his back. Not saying that others were, and I was probably being bossy about it. Stan had an effective game plan, and he executed it. Two things can be true at the same time: that a player is compromised, and that the other is out-playing him.