By winning the U.S. Open for the first time and completing his collection of grand slam titles, Rafa Nadal solved one riddle and posed another.

His 6-4, 5-7, 6-4, 6-2 victory over Novak Djokovic in Monday's rain-interrupted final at Flushing Meadows ensured the Spaniard joined the pantheon of tennis greats and resolved beyond any reasonable doubt just who is the best player in the world today.

Nadal had already won the Australian Open, French Open and Wimbledon singles titles and by adding the U.S. Open he joined Fred Perry, Don Budge, Rod Laver, Roy Emerson, Andre Agassi and Roger Federer as the only men to win the four majors.

At 24, he was the third youngest to achieve the feat and his best may be still to come.

He is the first man since Laver in 1969 to win the French Open, Wimbledon and U.S. Open in the same year and he now has the chance to hold all four concurrently if he can add the Australian Open in January.

His nine grand slam titles -- five French Opens, two Wimbledons, one Australian Open and now one U.S. Open -- took him to seventh place on the all-time list.

But the argument over who is the greatest of all time has once again been sparked and could rage for years.

Nadal is adamant Roger Federer deserves the accolade because of his 16 grand slam titles but the jury is still out after the Spaniard conquered New York.

Nadal has nine grand slam titles and is five years younger than Federer. He has been racking them up at a faster rate than his Swiss rival and has mastered every surface.

He is not walking the road to immortality so much as sprinting along it.

"For me, it's a dream to have the career grand slam," Nadal said.

"It is an unbelievable feeling because I worked all my life in difficult moments to be here."

With eight grand slams before the tournament began, the lefthanded Nadal was already one of the game's elite, but his win on Monday elevated him to a whole new level.

He overcame the distraction of a two-hour relay delay and losing his first set of the tournament to defeat Djokovic after three hours and 43 minutes of unrelenting tension at Arthur Ashe Stadium.

Djokovic, who had beaten Federer in an exhausting five-set semi-final on Saturday, expelled any doubts about his fighting qualities with a fearless performance, fending off an astonishing 20 break points during the course of the match, but the pressure eventually wore him down.

"I cannot hide the disappointment. I'm not gonna cry or complain about that. It's just the way it is," Djokovic said.

"I wanted that trophy, and I know I gave my maximum to get it even tonight, but tomorrow I will wake up as a new man."

After losing the second set on a sloppy service game, Nadal ran away with the last two -- clinching the victory when Djokovic hit a forehand wide -- and collapsed on court as the enormity of his achievement sank in.

He embraced Djokovic at the net then dropped to his knees again as the centre court crowd rose as one to applaud him. When he was handed the trophy, he raised it above his head and thousands of flashbulbs lit up a perfect New York night.

"For the first time in my career I played a very, very good match in this tournament," Nadal said.

"I played my best match in the U.S. Open at the most important moment."

Nadal has always seemed much older and wiser than he really is.

He could out-run and out-think most men when he was starting out on the professional circuit, but his relentless assault on the titles stalled last year because of a chronic knee injury that raised doubts about his future in the game.

The U.S. Open, played on hardcourts, had always been Nadal's biggest hurdle because of the wear and tear it puts on his aching joints.

Federer had won five times but has come up short in each of the last two years and at 29, his stranglehold on the event appears to be slipping.

Nadal arrived in New York fresher than ever and armed with a new and improved serve that enabled him to sail through his opening matches without fuss.

"Well, life changes sometimes. Ten months ago, (it) seemed like I was never gonna be the same. Now (it) seems I gonna be one of the greatest," Nadal said.

"I don't think it was that bad in that moment and not that good in this moment. It's always in the middle, no?