Rafa Nadal ran himself to the point of exhaustion in Monday's US Open final against Novak Djokovic then ran out of answers.

The Spaniard knew his best chance of beating Djokovic was to wear him down, so he chased everything he could to keep him on court as long as possible in the hope his Serbian opponent would run out of gas.

Part of his plan worked. Their final lasted four hours and 10 minutes and by the end, Djokovic was gasping for air and feeling his aches and pains.

But so was Nadal. He had run himself ragged by the fourth set and the result was now a foregone conclusion.

He gave it his best shot but it was not good enough. Djokovic won 6-2, 6-4, 6-7, 6-1, leaving Nadal to stand back and applaud as the winner raised the trophy.

"It was a tough match, physically, mentally, everything," Nadal said.

"I fought until the last point. I tried my best at every moment. I am happy with a lot of things, much happier than the previous matches against him."

It was another painful for loss for the Spaniard, who was on top of the world 12 months ago when he beat Djokovic in New York to complete his collection of Grand Slam titles.

Since then, the Serb has completely overwhelmed Nadal. They have played each other six times this year, all in finals, and the Serbian has won the lot.

The last time was at Wimbledon in July, when he also took away his number one ranking, leaving Nadal desperately searching for a way to beat him.

That defeat left Nadal deflated and disillusioned. His confidence was shattered and he arrived in New York with low expectations.

He may have lost to his great rival yet again but said he had come away from the defeat with a little bit of optimism, something he hadn't felt for a while.

"I had my chances, I really had my chances," Nadal said.

"You know what? I go back home knowing that I am on the way."

Nadal said he still had no real idea about how he could end his losing streak but was more motivated than ever to find a solution.

"I like to fight, I want to enjoy about this battle against him," Nadal said.

"Six straight loses, for sure that's painful but I'm going to work every day until that changes.

"So I have a goal. It's going to be tough to change the situation, but the goal is easy to see."

Although he lost his number one ranking and failed to defend two of his Grand Slam titles, he did retain his French Open title, Nadal said he could still look back on the year with a lot of satisfaction.

"Making the final is a fantastic result," he said.

"Last year, I won three Grand Slams, this year I won one but I played in three finals.

"So for me is a fantastic result. It's a dream result."