By Will Swanton
NEW YORK (Reuters) - Novak Djokovic believes Rafa Nadal may soon displace him as world number one and has plenty of time to break Roger Federer's benchmark 17 major titles after the Spaniard roared to the U.S. Open title at Flushing Meadows on Monday.
Djokovic was beaten 6-2 3-6 6-4 6-1 by Nadal, who claimed the 13th singles slam of his career to move ahead of Australia's Roy Emerson (12) on the all-time list.
Only Pete Sampras (14) and Roger Federer (17) are above the 27-year-old Nadal on the pecking order of major champions.
"Thirteen grand slams for a guy who is 27-years-old is incredible," Djokovic said. "I mean, whatever he has achieved so far in his career is something that everybody should respect. No question about it.
"I was saying before, he's definitely one of the best tennis players ever to play the game. Looking at his achievements and his age, at this moment - he still has a lot of years to play. That's all I can say."
Nadal seems certain to overtake Djokovic as world number one by the end of the year.
He missed the last half of 2012, and the 2013 Australian Open, because of a knee injury, which means he has no rankings points to defend.
Following his second major triumph this year, adding the New York crown to the French Open, only another injury or inconceivable loss of form will keep him at number two.
"What can I say?" Djokovic said. "He won so much this year. I'm still number one of the world in the rankings. But year to year he's far, far ahead. He has much more chances to end up as number one.
"Look, there is still tournaments to go. So we'll see."
Nadal's triumph gave him a 22-15 winning record against Djokovic and it was his sixth victory in their last seven meetings. The Serb conceded he had to find the key to reversing the trend in arguably the biggest rivalry in the men's game.
"I have to," Djokovic said. "It's part of my life. Many times you fall as an athlete. You have to learn the lesson and keep on going, keep on fighting, keep on improving.
"That's what we are here for. I'm still 26, and I believe the best time for my career is about to come. As long as I believe it, the fire and the love towards the game is inside of me, as long as that's present ... I'm going to play this sport with all my heart as I did in the last 10 years."
The Nadal-Djokovic rivalry is typified by marathon rallies that stretch both players to their physical limits.
They conjured a 54-stroke rally in the second set that left a capacity crowd inside Arthur Ashe Stadium screaming for more. Djokovic raised his arms at having succeeded in the mini-battle, but Nadal won the war.
"I have played against Rafa, on different surfaces and different occasions, points like this where you just feel that there is the last drop of energy that you need to use in order to win the point," Djokovic said.
"Sometimes I was winning those points, sometimes him.
"It's what we do when we play against each other, always pushing each other to the limit. That's the beauty of our matches and our rivalry, in the end."
For the second straight year, Djokovic started the season by winning the Australian Open before falling short at the other three majors.
He lost a five-set heartbreaker to Nadal in the semi-finals of the French Open and was beaten by Britain's Andy Murray in the championship match at Wimbledon.
"I wish I won at least one title more, considering the fact I played two finals," he said. "All the matches I lost, even the French Open, I had that match. I lost it again in the semis.
"Overall, it was again a very successful grand slam year for me. That's where I want to play my best in. As I said, I wish there was another title, but it is what it is.
"It was obvious that in the important moments Rafa played better tennis, and that's why he deserved to win. I congratulate him, and I move on. I didn't deserve to win in the end."