LONDON - Serbia's Novak Djokovic cemented his place as the world's top tennis player when he dethroned Roger Federer in an enthralling showdown at the ATP World Tour Finals, grinding down the Swiss to take the title 7-6 7-5.

Federer has been all but unbeatable in the cavernous O2 Arena since it began hosting the tournament in 2009 and was seeking a hat-trick of titles but despite some typical brilliance he was stopped in his tracks by the world number one.

The 17,000 crowd inside the Thames-side landmark created an electric atmosphere as a wildly fluctuating final unfolded.

Djokovic found himself 3-0 down in the opening set and 5-3 down in the second but the never-say-die Serb dug deep into his seemingly endless reserves of fighting spirit on both occasions before sealing victory in two hours 14 minutes.

It was his first title at the ATP's blue riband event since 2008 in Shanghai and he pocketed the $1.76m for going through the tournament with a 100 percent record having won all his round-robin matches.

Fittingly, his victory was his career-best 75th win of the year that while not quite as spectacular as 2011 when he won three of the year's four grand slams, was equally satisfying.

In 2011 when he won the Australian Open, Wimbledon and the US Open and took the world No 1 ranking for the first time he managed only 70 wins.

It was only the fourth time in the tournament's history that the world's top two players had contested the final and it did not disappoint.

Federer, seeking a record-extending seventh title, won the first nine points of the match on the way to a 3-0 lead.

The Swiss maestro, all smooth movement and silky skills, could hardly believe his luck as Djokovic struggled to find any sort of rhythm but it was not long before battle was joined.

Djokovic wiped off the deficit to level at 3-3 and then served for the first set at 5-4 having broken Federer in the ninth game when the Swiss was undermined by forehand errors.

Djokovic failed to convert a set point though and in the next game Federer drew blood, literally, when Djokovic tumbled to the blue-surfaced indoor court trying to retrieve a forehand.

He needed treatment on a cut elbow at the changeover but held for 6-6 and then edged the tiebreak 8-6 despite Federer saving a second set point after an incredible reflex volley.

Federer, cheered on by many fans waving Swiss flags, broke in the first game of the second set and looked set to take the last match of the season to the wire when he served for the set at 5-4, only to squander two set points.

Djokovic seized his chance, holding for a 6-5 lead and a deflated Federer could not hold the fired-up Serb off.