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It's not that Federer has lost anything. In fact, he's playing as well as he did in 2008, the last time he won this tournament. That said, something does seem to be missing.
It seems a bit of that winning attitude, that swagger which has really defined Federer throughout his heroically amazing career, may be gone.
Or at least it seems to have lessened to normal human levels. Even as he was winning Wimbledon in July, Federer seemed very low key compared to what used to define him.
He's always been a classy competitor and gracious in victory or defeat. But that on-the-court demeanor was characterized as fiery, whereas now it would have to be competitive. Either way it's just different.
Considering that Novak Djokovic, Andy Murray and even his fourth round opponent Mardy Fish are playing great tennis this week, Federer needs every advantage he can get to claim a record sixth US Open title.
Nothing should be easier to rekindle than that Federer swagger and attitude. He's just got find it within himself one more time.