I don't agree with Darth re: Wimbledon because it implies that a player, even one of Fed's caliber, shouldn't lose, ever, when he's that good. Even the GOAT is going to lose. You can't win all the time, as cliched as that sounds, because it ignores how sports and athletes work. They're not machines and they'll have an off year. Hell, even an off day costs you a tournament. So as good as Roger is on grass, seeing his level at the years in which he lost, I wouldn't say he underachieved at Wimbledon. Now if you argue that he should have been playing better, you're asking too much. He's a guy who was winning 3 slams a year routinely, so yes, he's going to experience a dip in level in his late 20's and early 30's, and it took him a while to fully adjust. Darth is almost literally asking him to be a machine.
However, Roger has underachieved at the US Open, no doubt about it. He should have won it in 2009 and really let that final slip from him by relaxing (I think it was a direct result of finally winning RG and breaking the slam record at Wimbledon). He even mentioned repeatedly that he wasn't that upset about it. Add to that the blown match points to Novak in back-to-back years, and you can definitely make a case for him underachieving. Now, I don't like the argument that "5 US Opens is too few for Roger" and leaving it at that. In theory, it isn't too few, but he should have won at least one more in 2009 and put himself in a position to win a couple of others in 2010 (although he wasn't beating Nadal that year) and 2011.
I'd also argue he would've been an underdog in the final in 2011 but it would have been close and definitely possible, as opposed to 2010 where he would have had no shot, and the general feeling was that Rafa was going to crush him (everyone was anticipating a Fedal final) due to how they had looked at that tournament, and how sub-par Fed had been since the AO.