In fact, US Open 2010, US Open 2011, Fed could have beat Joker (2 match points each time), but he chose not to because he knows that he will lose to Nadal in the final anyway. And he does not want to see Nadal to collect more titles and the only guy who can stop Nadal is Joker. Other examples are: Wimbledon 2011, how can he lost to Tsonga with 2 sets to love lead? And more recently is French Open 2012, he just chose to go out quickly to save Joker some energy, hoping Joker to defeat Nadal.
The Only exception is French Open 2011, I do not know why he wanted to beat Joker that time because if he chose to loose to Joker, with Joker's unbelievable form last year, Joker will definitely beat Nadal in the FO final last year. This year, he learned the lesson, but unfortunately Nadal is too strong on clay.
What I am saying is true, because Fed cares about his status in tennis history very much and he knows Nadal is his biggest threat and he is 2 out 10 in grand slam against nadal, that means he only wins 20% of the time in a grand slam against Nadal.