I don't really agree. It happened to Rafa with the kind of flat hitters he always had trouble with, and not always, just in certain matches he was losing without not many chances of recovering and she seemed rightly annoyed or sad. Also it's usual to see Nadal starting a bit out of focus in many matches, he's kinda slow starter and it's not unusual to see him losing a first set or at least losing a number of games at the start.
But I don't think I've seen Nadal that nervous in late stages of a match he had good chances of winning, as yesterday. He's a clutch player, that's one of his main traits and he's known for that. Maybe, and now I'm just making a hypothesis here, he was a bit intimidated by Djokovic's streak, or by the perceived importance of having to prove he's the #1, or something like that.
It wouldn't be strange, it's what happened to Federer when Nadal got into his head. In Federer's prime, I never saw Roger so intimidated against any other player. Now I think I saw this in Nadal. I could be wrong though.
I would agree if not for the example of Davydenko. Rafa always looks flustered against the russian's gameplan and has the same defeated look once he gets a foothold in the match only to let it go. Remember, Rafa lost a final to Davy after BAGELING him and arguably playing better than yesterday against Djoker. And davydenko isn't gunning for nr.1 spot.
Rafa did not look flustered at all yesterday for like a set, even though he was playing average tennis as well. He only started to look like that once Djoker brought EVERYTHING in. I was staring at the screen at times and it was like Nadal from 2005 on clay, NOTHING was getting past Djoker past both wings. Nadal is human and this affected him. When you have to play 20+ just to get a point, it messes with your head. Couple that with some parts of the Nadal game that did not work(like his BH ROS which was dreadful) and you can see how you lose your cool.
Remember what happened in IW. Rafa was simply dominating off the ground, playing much better tennis that in yesterday's first set and then his serve faultered. His whole game was thrown by this, including his mental ability. Nadal said that all he could think of from the 2nd set onwards was getting a first serve in.
The point I am trying to make is that Djoker isn't doing some kind of special mental pshyche out to Rafa, he is doing what top guys do to him on HC, keep him in the back of the court and make him doubt his game. Rafa gets more defensive, passive and being bullied around makes his mental ability go down.
Rafa needs to be near perfect on most aspects of his game to win HC titles or top matches against HC foes. Remember USO 2010 final. He can win clay on grass with his B game but that does not work on HC. This by now is pretty clear.
If you wanna look at nervousness, look at Rafa in the Berdych match or against Karlovic, even if those weren't in finals.
Also Rafa is starting to have second set dips just like Fed.