Novak Djokovic has certainly left his mark over the past couple years in the ATP. In his last seven Grand Slam appearances, Djokovic has been in the quarterfinals every time, the finals three times and won twice.
He's lost only two matches in all of 2011.
Here we are at the 2011 US Open, and there is reason to worry about how well Djokovic will perform. Yes, he did win his first-round match against Conor Niland, but his opponent retired. Why do I believe there is cause for concern?
Here's five answers to that question.