An inspired Novak Djokovic beat world number one Rafael Nadal 6-3 6-4 at the O2 Arena in London to retain his Barclays ATP World Tour Finals title.

After losing the US Open final to Nadal, Djokovic has won 22 straight matches.

The three-time defending Australian Open champion will now head into 2014 halfway to bettering his incredible 43-match unbeaten run in 2011.

Nadal lost his only other final at London's O2 Arena to Roger Federer three years ago, and it remains the one significant title he has yet to win.

Nadal had hoped to cap one of the most stunning years in tennis by winning the one major trophy still to elude him, and he said: "It's been no doubt the most emotional season of my career.

"To play the last match in this stadium is a fantastic occasion for me even if I didn't have the chance to win today because I played a fantastic player like Novak."

It was fitting that the season was brought to an end by a 39th meeting between the two standout players of it.

A packed crowd lapped up the latest instalment of the Open era's most prolific rivalry - Nadal led 22-16 overall and 3-2 this season.

Neither man had been beaten at the O2 this week but the indoors arena seemed likely to give Djokovic the advantage and he set about showing that, exploiting some early hesitancy from his opponent to move into a 3-0 lead.

It could have been four straight games but Djokovic missed a makeable backhand and a roar of annoyance told just how important he thought that was.

Both men looked nervy and some wild shots from the defending champion helped Nadal break back for 2-3.

Double faults were really hurting the Spaniard, though, and a fourth of the match gave Djokovic a second break point in the eighth game.

Nadal had saved the first with an excellent serve, and he thought he had done the same again but Djokovic somehow scrambled a lob off a volley and roared long and loud when he eventually won the point.

That left him serving for the set and he clinched it with a second ace.

Nadal was simply not managing to hit winners against his fellow defensive master, but it was Djokovic's attacking that forced the error from the Spaniard as he conceded another break in the third game of the second set.

Djokovic's volleying was one of the most impressive parts of what in truth was not a great match, and more excellent net play gave him two chances to lead 5-2, but this time Nadal was up to the challenge.

The top seed then saved a match point with a fine backhand and kept Djokovic waiting for a bit longer when he won a lengthy rally to save a second, but on the third chance Nadal drove a forehand just wide.

There was success for Spain in the doubles, though, with Fernando Verdasco and David Marrero beating American twins Bob and Mike Bryan 7-5 6-7 (3/7) 10-7.

Marrero broke down in the trophy ceremony as the pair dedicated their win to his grandfather, who died two years ago to the day.