By Larry Fine

NEW YORK (Reuters) - World number one Novak Djokovic pronounced himself fit for the U.S. Open on Monday and eager to take the next step after a spirited run to last year's final served as a springboard to his sensational 2011 season.

Djokovic, who suffered just his second defeat of the year eight days ago when he retired from the Cincinnati Open final because of pain in his right shoulder, said he feels fine even after hitting balls for about 90 minutes on Sunday.

"I have been serving in the last couple of days, playing 100 percent, so I'm ready for the tournament," the 24-year-old Serbian told reporters. "Right now my shoulder is in really good condition. I didn't feel any pain, I don't have any inflammation right now."

The Australian and Wimbledon champion said his rousing victory in last year's U.S. Open semi-final against Roger Federer and his pulsating final against Rafa Nadal gave him the confidence to lift his game.

The top-seeded Djokovic, who plays his first-round match Tuesday against Conor Niland of Ireland, said psychology has played a pivotal role during his career.

"I was always coming to the later stages of a grand slam, semi-finals, but I wasn't managing to make that extra step, because I didn't get that positive mindset on the court when I played the big guys," he said. "Now I changed."

But while Djokovic said he felt on top of the world both on and off the court, he expected to have to keep fighting to hold onto his number one status in tennis.

"The fact is that they have been the two most dominant players in the world," said Djokovic, who on Monday was named a UNICEF goodwill ambassador promoting the rights of children in Serbia. "Even though I'm still number one, they are the two most successful players that there are active in today's tennis."

(Editing by Frank Pingue)