NEW YORK (Reuters) - Serena Williams, through sheer force of will as much as her skill with the racket, beat world number one Victoria Azarenka 6-2 2-6 7-5 in a dramatic U.S. Open final on Sunday to capture her 15th singles grand slam title.

Serena Williams of the U.S. poses with her trophy after defeating Victoria Azarenka of Belarus in their women's singles finals match at the U.S. Open tennis tournament in New York September 9, 2012. REUTERS/Kevin Lamarque

On her best behaviour but struggling to subdue an opponent seven years her junior, Williams provided an everlasting reminder of her incredible fighting qualities as she became the tournament's oldest women's champion in nearly four decades.

Already one of the greatest players to grace the game, the American's longevity adds new credence to her place in the sport's pantheon with Sunday's victory coming almost 13 years to the day she won her first U.S. Open as a teenager in 1999.

"I don't consider myself the greatest," she said.

"I just consider myself a tennis player who's trying to do the best I can."

Her clash, against the top player in the world this year, lived up to all expectations and more as the pair traded blows for almost two and a half hours at Flushing Meadows.

Azarenka, who won her maiden grand slam title at the Australian Open in January, led 5-3 in the deciding final set and was within two points of victory when Williams lifted and reeled off the last four games to capture her fourth U.S. Open crown.

"I have no regrets. I felt like I gave it all there," said Azarenka.

"Could it have gone my way? Probably, yes. But it didn't and it really, really hurts."

SIGNS OF NERVES

The opening exchanges gave few clues of what was to come as Williams romped through the opening set in 35 minutes with two service breaks and 16 stunning winners.

But the Belarusian returned to the court with renewed determination and found a way to penetrate her opponent's serve.

As Williams started to show the first signs of nerves, Azarenka seized her opportunity, breaking her serve twice, and for the first time since 1995, the women's final at Flushing Meadows was forced into a winner-takes-all third set.

The capacity crowd at the bear-pit that is the Arthur Ashe Stadium centre court roared with excitement as the two combatants ran themselves to the point of exhaustion, producing some breathtaking shots as the stakes rose.

Williams kept her cool, even when she was called for a foot fault, avoiding a repeat of her petulant behaviour at Flushing Meadows in the 2009 semi-final against Kim Clijsters and last year's final with Sam Stosur.

After facing serious health issues, including the discovery of a life-threatening blood-clot on her lung, in the past 18 months Williams has developed a new perspective about her actions on court.

"I really think a champion is defined not by their wins but by how they can recover when they fall," she said.

"I have fallen several times. Each time I just get up and I dust myself off and I pray and I'm able to do better or I'm able to get back to the level that I want to be on."

THE 'GREATEST'

In the third set, it was the American who blinked first and her golden summer, which included winning a fifth Wimbledon title and two gold medals at the London Olympics, looked like coming to an end.

She was two points away from losing but regained her composure when it mattered most by winning the last four games then collapsing on the court at the realisation she had won again, with little more than two weeks before her 31st birthday.

"For me, she's the greatest player of all time. She took the game to the next level," Azarenka said.

"I congratulated her with all my heart because I felt like she absolutely deserved the win. She was the best player out there today."

Only Australia's Margaret Court, who had already turned 31 when she won her last U.S. Open crown in 1973, won the title at an older age than Williams but the American is not finished yet.

"My motivation is so up there," she said. "I just feel like I'm ready for the next tournament.

"I really want to be focused and do well there and just keep the dream alive."

(Editing by Ian Ransom)

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