Upon the withdrawal of defending U.S. Open champion Rafael Nadal on Monday from the tournament, Novak Djokovic, Roger Federer and Andy Murray are the most possible contenders of winning the crown.

Djokovic, the World No. 1, won the event in 2011 and has placed second to Murray and Nadal in 2012 and 2013 respectively. He is the recent Wimbledon winner, edging out Federer in a classic five set match.

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Djokovic, 27, hasn't been playing good tennis after his win at Roland Garros. Exiting early in the third-round in both the Rogers Cup and recently concluded Western and Southern Open.

The Cincinnati Masters is the only Masters Series event that top seed Djokovic has not won as he was defeated by Spaniard Tommy Robredo.

The 17-time Grand Slam champion Federer's last Grand Slam title was at Wimbledon in 2012. He last won the U.S. Open in 2008, but Federer is playing well lately, winning the crown in Cincinnati and placing second to Canada Open winner Jo-Wilfried Tsonga.

Britain's No. 1 Murray, on the other hand, last won the tournament in 2012 and has reached the quarter-finals at Wimbledon, losing to Bulgarian Grigor Dimitrov.

Murray, 27, however, has not been impressive in both tournaments in Toronto and Cincinnati, exiting in the quarterfinals after losing to eventual winners Tsonga and Federer respectively.

On a coach's point of view, Murray is still grieving from the split-up with Ivan Lendl and adjusting to Amelie Mauresmo.

Djokovic's coach is six-time Grand Slam champ Boris Becker while Federer's mentor is also a six-time Grand Slam champion, Stefan Edberg, who has been coaching him since January.

Becker and Edberg are bitter rivals during the mid-80s up to the mid-90s. Becker has won 25 times against to the 10 of Edberg against each other but is tied in Grand Slam tournaments with two apiece.

The tournament in New York is on a hard court surface. Edberg is a little better than Becker in hard courts winning the US Open twice compared to the one-time of Becker.

Federer has won the US Open five straight years while Djokovic only won the event once.

The 33-year-old Swiss is most likely the candidate to win the prestigious crown in Flushing Meadows but could end up with a possible Wimbledon rematch against the Serb.

Tagsrafael nadal, Novak Djokovic, Roger Federer, andy murray