Novak Djokovic of Serbia reacts after winning match point against Rafael Nadal of Spain during the men's final of the US Open at the USTA Billie Jean King National Tennis Center on Monday. Jared Wickerham / Agence France-Presse

Novak Djokovic, whose US Open triumph was his third Grand Slam title of the season and 64th win in 66 matches in 2011, insisted on Monday that he is not invincible.

But his modesty did not prevent the 24-year-old world number one from keenly anticipating his next big ambition in a sport he is beginning to monopolize - a career Grand Slam.

Rafael Nadal, whom he beat 6-2, 6-4, 6-7 (3/7), 6-1 in a gruelling, brutal final, was the last man to win all four Grand Slams when he triumphed at Flushing Meadows in 2010, to become the seventh man to achieve the feat.

Djokovic missed out on a rare calendar Grand Slam in 2011 when he lost in the French Open semifinals to Roger Federer, a defeat that ended his 43-match winning streak.

But despite dominating 2011, winning three majors and 10 titles in all, including five Masters, Djokovic is desperate to keep predictions in check.

"I am not invincible," said Djokovic.

"I just think a positive attitude keeps you on top of your game, when you go on court and when I had the start to the year that I had. Of course I want to win more majors and prove things to myself. It would be unbelievable to complete the Grand Slam. I need to win the French Open, but I think it will take time."

Despite Djokovic's caution, he still has many backers including American legend John McEnroe whose all-time season-best record of 82 wins against just three defeats, set in 1984, is now well within the Serb's sights.

"He has had the greatest year in the history of our sport," said McEnroe on Sunday.

The statistics back up McEnroe's conclusion.

Among Djokovic's 64 wins are six in six matches against Nadal - all in finals - four wins against just one loss to Federer and two wins and one defeat against world number four Andy Murray.

Even that loss to the Scotsman in Cincinnati was due to an injury retirement when Djokovic damaged his shoulder.

Despite his heroics and the belief that leading Serbia to the Davis Cup last year was the catalyst to his successes, Djokovic also believes the seeds of his victory in New York were in the first round last year.

He had to come from a set down to defeat compatriot Victor Troicki in a duel that taught him many things about his game, lessons that became important when he saved two match points in the semifinals against Federer before finishing runner-up to Nadal.

"Last year I made the final but I could have easily lost in the first round. I overcame that. I needed those tough matches to get confidence and the belief that I could win the majors.

"Something just clicked. In the previous years, I hadn't changed my game in any big way and my strokes were still the same and was hitting the shots I wanted to hit.

"But I had difficulties approaching semifinals and finals. I would wait for players to make mistakes. I didn't have the positive attitude. That has changed now - the 2010 US Open was the turning point."