Look, I'll only answer you since you're the only one coming up with decent points and actually addressing the situation rather than the others who only throw in smileys and insults.
First of all, you can be the best clutch player, and still be affected by nerves. They're all human, like one performance like this instantly nullifies his clutchness. All players have had weaker moments, Nadal has overall had the fewest of the players on tour and a lot of strong ones. We know how often he doubts himself, but usually that doesn't translate onto court with him. At IW, it did. That doesn't make him a mental midget of course.
Remember, Nadal was in tears at the Australian Open, failing to complete the Rafa slam, losing to Ferrer due to injury. In my point of view (which is supported by later interviews) he felt like that slam would've been his if it wasn't for that. That was the first (of many) devastating blow(s) in 2011. He then returned to tour at IW having to prove something. There were already claims of Novak being 'the real #1' and I think he fixated too much on that, got tight in set 2 and the rest is history.
I am sorry but this just doesn't make sense and you're thinking too much. Truth is, in that final IW 2011, if anyone had anything to prove, it was Nole. He was only a 2 slam champion, who had just won his second slam THREE years after the first one. Many people back then were saying that it had been a second fluke on a surface that favors him, so he was never going to win any other major. He was also already the better hard court player of the two, who had had an impressive start of the season with 2 titles but hadn't beaten the world number 1 YET. So the way I see it, if anyone was under real pressure that day, it was Nole.
You simply can't say that rafa crumbled that match because he was under pressure. He wasn't, not that early in the year.