Serena Williams sailed into Sunday’s U.S. Open final with such ease it was impossible to know just how much fire still resided in the 17-year veteran of the pro tour.

Victoria Azarenka found the answer, denying Williams both times she served for the victory in the second set with the season’s final major at stake. Azarenka stormed back from a 1-4 deficit to win the tiebreak that forced a decisive third set.

But Williams never lost her composure in a match bedeviled by a capricious wind and packed with brilliant shots and courageous bounce-backs by both players. Instead, she relied on her strongest weapon, her vaunted serve, and big-match savvy to vanquish the world’s No. 2 player, 7-5, 6-7 (6-8), 6-1.

Only upon clinching the match 2 hours 45 minutes after the first ball was struck did Williams let her pent-up emotion fly. Less than three weeks shy of her 32nd birthday, Williams jumped up and down like a manic toddler, raised a clenched fist and shouted, “Come on!” “Come on!” even though her wearying work-day was over.

Both players were rewarded with a standing ovation and wild applause by the 24,500 at Arthur Ashe Stadium, former president Bill Clinton among them.

After regaining her poise, the triumphant Williams paid tribute to Azarenka, whom she beat in a three-set thriller in last year’s U.S. Open final, as well, as a great opponent and great fighter.

Azarenka, 24, returned the praise as she fended off tears.

“To be in the final against the best player in the world, who deserves to win, it’s incredible,” Azarenka said. “We showed our hearts. We gave it everything we got.”

With the victory, Williams claimed her fifth U.S. Open title and 17th Grand Slam, one shy of the 18 held by Chris Evert and Martina Navratilova. Both veterans said this week they fully expect Williams to surpass them.

With Williams winning four of the past six majors, and proving herself at the peak of her powers, suddenly the Open Era record of 22 held by Steffi Graf seems within her grasp, as well.

Sunday’s triumph was worth $3.6 million to Williams, a record for a tennis player. And it pushed her career prize money over $50 million.

But if Azarenka proved anything in defeat, it’s that she’s the player most likely to stand in Williams’s way going forward.

“I can’t wait for the next chance,” said Azarenka, who broke Williams’s serve four times, twice as many times as Williams’s six previous opponents combined. “The challenge that is in front of me is only going to make me more motivated.”

It was the first time in 10 years that the No. 1 and No. 2 women had met for the U.S. Open title, and it held the promise of a terrific fight. Williams entered with a 66-4 match record this season, but Azarenka accounted for two of those defeats.

The match got underway on a gorgeous afternoon, but the gusting wind addled both players, throwing off ball tosses, footwork and timing while buffeting Williams’s flouncy dress around her waist.

Serving a must-hold game at 4-5 in the opening set, Williams double-faulted, was called for a foot-fault and couldn’t run down a cagey drop-shot. But she held serve in the nearly 10-minute game, then blasted a forehand winner to get the critical break on the next game.

From that point on, Williams seemed to stop fighting the wind and became a study of absolute concentration, jaw clenched, as if a tsunami couldn’t jar her focus. And she produced her best tennis to that point, claiming the opening set by holding at love.

Williams’s tear continued in the second set, breaking twice to take a 4-1 lead.

But Azarenka dug in, denying Williams both times she served for the match — at 5-4, in which she was as three points from the title, and again at 6-5. Whether it was the wind, nerves or a combination, Williams over-hit too many balls. A tiebreak was needed to settle it.

That was a mini-drama of its own, with Williams striking for an early lead, then coughing up the advantage with a series of errors. It took Azarenka three set points, but she won the tiebreak to force a decisive third set.

It was the first set that Williams conceded all tournament, but she resisted the urge to berate herself for not closing the match in straight sets.

“I thought, ‘I have to just relax, calm down and play smarter tennis,’ ” Williams recounted.

The turning point came in the fourth game of the final set, when Williams broke Azarenka for the sixth time. And she didn’t look back, claiming her fifth U.S. Open title 14 years after winning her first.

“She is a champion, and she knows what it takes to get there,” Azarenka said of Williams. “I know that feeling, too. And when two people who want that feeling so bad meet, it’s like a clash. That’s what happened out there.”