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American great Serena Williams outlasted world No. 1 Victoria Azarenka on Sunday to capture her fourth U.S. Open title.

A now 15-time Grand Slam winner, Williams won the final four games in the third set to pull out a 6-2, 2-6, 7-5 victory over Azarenka at Arthur Ashe Stadium.

"I can't believe I won," Williams said. "I came in the summer knowing it was going to be a long summer, but I knew that I could do well if I just put my mind to it. I knew that I could just be a good player and a champion this summer. I never expected to win all these titles. Everything has been so amazing."

The fourth-seeded Williams improved her record to 4-2 in championship matches at the year's final major. She was a runner-up to Aussie Samantha Stosur last year.

Williams fired 13 aces past the Belarusian Azarenka, who was appearing in her first-ever U.S. Open final. The American had 31 more winners (44-13), but also committed 17 more unforced errors (45-28).

Azarenka seemed poised to win her first U.S. Open crown after hitting a solid forehand winner down the line for a 5-3 lead in the third set.

Following a hold from Williams, Azarenka had a chance to serve for the match. But she found herself down 0-40 before firing a forehand into the net to give Williams the break.

After another hold from Williams, the pressure was on Azarenka to stay in the match. A backhand from Azarenka went into the net to set up deuce. The Belarusian put two shots past the baseline to give Williams her fourth U.S. Open title.

Williams fell to the ground in disbelief after watching Azarenka's final ball sail long. The American jumped for joy on the court as she thanked the crowd.

"Obviously, I would have wanted to win easy. But at the same time, this is more exciting to win because you don't really know what's coming," Williams said. "You don't know what to expect, and then you get it. This is the best feeling I think in tennis."

It was the first U.S. Open women's final that went three sets since Steffi Graf defeated Monica Seles in 1995.

Azarenka appeared in her second career Grand Slam final, with both coming this year. In a rematch of this year's Australian Open final, Azarenka overcame former world No. 1 Maria Sharapova in the semifinals on Friday.

"Definitely. It's a great achievement, there is no doubt," Azarenka said. "It's kind of difficult to sink everything in at this particular moment, because right now I feel sad. I feel proud of myself in one way, but still sad. But in few days when I go home, you know, I'll be more than happy, you know, with the summer. I think I'm in pretty good shoes, you know, sitting here as a finalist of the U.S. Open actually for my first time."

Williams handled 10th-seeded Italian Sara Errani in the semifinals.

Williams, the reigning Wimbledon champion and Olympic gold medalist in singles and doubles, improved to 10-1 lifetime against Azarenka.

The women's final was pushed back until Sunday because of weather issues on Saturday. The men's final between Novak Djokovic and Andy Murray will be played on Monday.