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Novak Djokovic put a firm stamp on his position as world number one with a stunning victory over Roger Federer in the ATP World Tour Finals.
The Serb won 7-6 (8-6) 7-5 to add the season-ending title to his Australian Open victory and top ranking.
He needed two hours and 14 minutes to take two sets in a match of dramatic swings in momentum.
It is the second time Djokovic has won the ATP finale, following his triumph in Shanghai four years ago.
"I was just trying to hang in there," said Djokovic.
"It's not the first time that Roger started so well against me. I've experienced before his aggression, really trying to put his mark on the match."
He added: "Whenever I needed to come up with some really good shots - really focus myself and get every ball back in the court - I did that, so I cannot be more thrilled than now."
Federer was attempting to become the first man since Ivan Lendl 25 years ago to win a hat-trick of season-ending championships, and his seventh overall.
"Maybe there's a bit of regret because I had the lead twice before him," the 31-year-old admitted in defeat.
"At the end of the day that doesn't matter, you have to get over the finish line in the set and then obviously the match. He was better at that today.
"What he does well - even in defence he stays somewhat offensive. That, I think, is what separates him from the rest a little bit."
Federer has made London's O2 Arena a second home in winning there for the last two years and, as ever, the Swiss did not lack for support.
The atmosphere might not have been cranked up at the start as it was in Sunday's semi-final against Andy Murray, but the Federer fans were in full voice as their man reeled off the first nine points.
Djokovic appeared nervous, fluffing a routine volley and firing long as he slipped, and there were ironic cheers when he finally got on the board in game three.
The winners were flowing from Federer but it was a case of brilliance against resilience as Djokovic dug in and weathered the storm by extending the rallies whenever possible.
Federer netted a forehand to give back the break in game five and the contest came alive.
Djokovic hit a brilliant cross-court forehand winner when tumbling to his right to move 5-4 ahead, but he failed to convert a set point on serve and Federer hit back to level at 5-5.
The heavy blows took a physical toll when Djokovic was knocked off his feet by a Federer effort and needed attention at the changeover, but the hour mark passed and the pair headed to a tie-break.
Djokovic took the initiative in it, but when Federer played a spectacular forehand winner, almost from behind him, he punched the air and brought the O2 crowd to its feet.
The only person apparently unaffected was Djokovic, who moments later clinched the set anyway with a forehand winner after a Federer error.
An eight-minute game at the start of the second set suggested there was still nothing between them, and when Djokovic succumbed on the fourth break point he smacked his racquet in anger on the blue court - another long battle loomed.
Federer resumed his early dominance, and when he saw off a break point at 4-3 with an ace he looked on course to force a decider.
Djokovic had recovered from worse positions against Murray and Juan Martin del Potro in the last week, however, and Federer helped him out when serving for the set at 40-15.
His forehand deserted him and two set points went begging as Djokovic, now peppering the baseline with his returns, reeled off four straight points to level.
Having been on the verge of levelling the match, Federer was now hanging on for dear life.
Djokovic battled through his own service game and a couple more loose shots from Federer, under pressure from the Serb's returns, offered up match point, seemingly from nowhere.
The final point was played out much like the match, with Federer taking control only for Djokovic to come up with the answer - a spectacular backhand down the line on the stretch.
It was a breathtaking shot on which to end the tennis year, and one that summed up why Djokovic remains number one among such star-studded company.
"It's the best way to finish a match, I guess, with a passing shot - one of my favourite shots," he concluded.