NEW YORK (WFAN/AP) — Sam Stosur could feel her heart pounding as she sat in her chair after winning the first set of the U.S. Open final.

“I’ve got a chance to win one out of the next two,” she thought.

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Turns out she wasn’t giving herself enough credit. The Australian won the second set almost as convincingly as the first to earn her first Grand Slam title, beating 13-time major champion Serena Williams in 73 minutes Sunday.

“If I was going to win, I didn’t think that I’d be able to do it in 6-2, 6-3, that’s for sure,” Stosur said.

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Maybe that’s because she hasn’t always done things the quick and easy way in her career. Like missing nearly eight months in 2007-08 because of Lyme disease.

“I’ve slept in train stations and stayed in dodgy hotels and done the hard yards through many places,” the 27-year-old said.

In this tournament alone, Stosur set two records of the long and dragged-out variety. She needed 3 hours, 16 minutes to win in the third round — the longest women’s match at the Open since the advent of the tiebreaker in 1970. Two days later, Stosur was part of the longest tiebreaker in a women’s Grand Slam, 32 points in another three-set victory.

Stosur had struggled with inconsistency since reaching her first major final last year at the French Open. That day, Stosur was the favorite — seeded No. 7 to 17th for Francesca Schiavone. The Australian lost a tight match in which she was up a break in the second set.

This time, Williams was the overwhelming favorite.

“I felt like I was definitely the underdog going into it, so maybe that kind of made me a little more relaxed going into this match than especially my last Grand Slam final,” the ninth-seeded Stosur said. “I think I was able to draw on a lot of that experience from the French Open.”

She looked cool and calm from the start, going after Williams’ second serve and smacking winners with her forehand.

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“She was cracking ’em today,” Williams said. “She definitely hit hard and just went for broke.”

Stosur has worked with an Australian sports psychologist for the last two years, and she especially needed that after getting off to a slow start this season.

“At the end of the day I had to believe that I was going to feel like I could play my best tennis again, and to realize that it’s not all gone,” she said.

She couldn’t play better than she did Sunday. It helped that Stosur had beaten Williams twice in her career, and after six meetings in all, she was no longer fazed by Williams’ intimidating presence.

Stosur used that composure when Williams lit into the chair umpire over a point penalty in the first game of the second set.

“It was probably the loudest I ever felt a crowd in my whole entire life,” Stosur said. “You’re right in the middle of it. It was definitely a quite overwhelming feeling. But once I hit that next ball in the court and started playing again, I felt settled.”

Williams broke back in the next game, but Stosur took control to win the last four games of the match.

“I’ve kind of surprised myself with how much I have been able to mentally stay focused on court and bounce back from the adversity from matches,” Stosur said.

She became the first Australian woman since Evonne Goolagong Cawley at 1980 Wimbledon to win a Grand Slam title. Goolagong Cawley sent Stosur a text message saying, “Twinkletoes, you finally have got what you deserved.”

“Hopefully,” Stosur said, “this is the first day of a new beginning for me as a player.”

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