Also think the points about Djoker's body language/attitude are valid in this discussion. With the exception of the last two Aussie Open's, this has been a common them with him. Random lapses in huge slam SF's and F's.
He looked out of the 2012 French SF vs. Rafa, then got going when the rain came, but ultimately lost.
Looked lackluster in his SF loss to Fed in the 2012 Wimbledon.
Let the windy conditions take him out of the match in the first two sets of the 2012 USO final.
Again, had a couple of lapses in the 2013 RG final that cost him a chance at the first two sets he lost in that match, not to mention the net touch in the fifth set that allowed Rafa to get back even.
Then, this final he looked disengaged from the start and never did really get/seem into the match.
Will he be motivated and focused enough for the USO?
It his human nature to struggle with motivation and focus when you've reached the top of the mountain. I think that point, and Djokovic's example in particular, underscore just how impressive and amazing Federer and Nadal are and have been for maintaining that fire for such an extended period. Both guys have been well-documented but...
Federer - 237 consecutive weeks at No. 1, a stretch where he made 18 of 19 slam finals, 23 straight SF's, 36 straight QF's, etc.
Nadal - Wins everything on clay, never a letdown, 8 RG titles (only one loss ever there), 8 Monte Carlo's (or is it 9?), there's a reason no one has ever won a slam 8 times until now
Even if you are fortunate enough to have the physical ability to play at that level for an extended period of time, it is so difficult to sustain the necessary motivation and focus to repeat that success time after time on the court. These two guys have made it look so routine for so long, I think we've lost perspective on just how amazing they've been. I think some of us, after 2011, were just expecting Djokovic to carry that mantle forward, roll off 2-3 more multiple slam seasons and get in the discussion with Rafa, Pete, Roger and Borg. But, its not that simple and its not that easy.