U.S. Open: Serena Williams 'pumped up' by defeat to Victoria Azarenka

(CNN) -- Serena Williams has lost only four matches this year -- and the world No. 1 is hoping her most recent setback can help her become the oldest winner of a U.S. Open women's singles title.

The American is favored for a fifth triumph at Flushing Meadows, but this month's defeat by second-ranked Victoria Azarenka has given the Belorussian hope of avenging last year's final heartbreak.

Azarenka served for the title just under 12 months ago in New York, but Williams fought back to claim her 15th grand slam title.

She has since added another, at the French Open in June, but suffered a shock reverse against Sabine Lisicki in the last 16 at Wimbledon.

Williams added two more WTA Tour titles at Bastad and Toronto, but was upset by Azarenka in a third-set tie-breaker in the Cincinnati final.

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"Every time I lose, I get so pumped afterwards," said Williams at a pre-tournament press conference Saturday.

"I just feel like now I'm ready, now I'm prepared. I almost needed that to take my game to a new level."

Williams will be 32 on September 26, and if she can retain her title in the final 18 days before that then she will surpass Margaret Court's 1973 effort of being the tournament's oldest woman winner.

"I have been able to keep up with the times," Williams said. "I'm actually serving harder than I ever have in my career. The racquets are stronger and I'm more fit.

"I feel so good, so healthy, so vivacious every time I step on the court."

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With world No. 3 Maria Sharapova missing the tournament due to injury, and Wimbledon champion Marion Bartoli having retired earlier this month, Azarenka is Williams' biggest threat along with Polish third seed Agnieszka Radwanska.

Williams will face former French Open winner Francesca Schiavone in her opening match Monday, while older sister Venus plays Belgium's Wimbledon semifinalist Kirsten Flipkens.

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Azarenka will begin the two-week event against Germany's grand slam debutant Dinah Pfizenmaier.

"You can always take the best out of what happened last week, so I will definitely take that into consideration," the 24-year-old told reporters Saturday.

"But the new week, U.S. Open, Serena, number one player in the world, defending champion, we all start kind of from zero here."

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In the men's draw, Rafael Nadal and Roger Federer will both be in action on Monday.

Second-ranked Nadal will face American Ryan Harrison as he bids to win the hard-court event for the second time. The Spaniard was champion in 2011, then runner-up the following year, but missed the 2012 event due to injury.

Federer, the champion five years in a row from 2004, will play Slovenia's Grega Zemlja as the seventh seed following a difficult year in which the 17-time grand slam winner has struggled with both injuries and an experiment with a new racquet.

Meanwhile, world No. 9 Petra Kvitova's U.S. Open preparations stumbled Saturday as the Czech lost 6-2 6-2 to Romania's Simona Halep in the final of the WTA event in New Haven.

Halep, a quarterfinalist in Cincinnati, followed up Friday's win over former No. 1 Caroline Wozniacki to claim her fourth title this year.

France's Gael Monfils may miss New York after suffering an abdominal injury in the final of the ATP's Winston-Salem tournament.

The former world No. 7, now ranked 49th after a series of physical setbacks, retired at 6-3 2-1 down against Austria's Jurgen Melzer.