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Tennis is a sport where the mental side is almost as important as the physical side. No matter how talented a player might be, he will not be the world's best if he's not strong mentally. In fact, Djokovic himself admitted that his emergence as the best player in the world has a lot to do with a better mental focus.
As mentioned before, there are four players right now who are head and shoulders above the rest. Djokovic has a mental edge against all of them. Andy Murray is yet to win his first Grand Slam and he was mercilessly crushed by Djokovic in this year's Australian Open Final, Nadal has lost his five consecutive matches and his no.1 ranking to the Serb, and Federer, despite being the only man to defeat Djokovic this season, has lost his last two hard court Grand Slam matches against the Serb. Djokovic has a clear mental edge over his three closest rivals.
That said, it'd be foolish to leave Juan Martin del Potro out of the discussion. The 2009 champion basically didn't compete in 2010 due to injury, but he has been slowly finding his game again and made life very, very difficult for Rafael Nadal at Wimbledon. If he manages to reach top form, he might potentially be Djokovic's main challenger in New York. His powerful serve and groundstrokes make him almost unstoppable in Hard Courts when on form.
The differences at the top of the game are so little that Djokovic's superior confidence might just make the difference at the US Open. Right now, he believes he can beat anyone, any time, a belief that has been fueled by his impressive results this year.