NEW YORK -- That Maria Sharapova's shaky serving contributed to her early exit at the U.S. Open -- to the tune of a dozen double-faults -- came as no surprise. She's faced that problem for quite some time.
That Sharapova's other strokes also were problematic Friday could be explained away by the perpetual motion of her opponent in the third round, 26th-seeded Flavia Pennetta of Italy.
That three-time major champion Sharapova's nerve would fail her in the crucible of a third set? Now that was the real stunner.
Unbeaten this year in 12 previous matches that went the distance, the third-seeded Sharapova faltered down the stretch and dropped the last seven points of a 6-3, 3-6, 6-4 loss to Pennetta that took 2½ hours.
"She's a good fighter, you know. You can never give up with her. You have to be focused until the last game; until the last point, actually," Pennetta said. "But I think (at) 5-4, she's starting to feel a little bit of pressure."
After trailing 3-0 and 4-1 in the last set, Sharapova turned things around briefly, getting to 4-all, 15-30 on Pennetta's serve. But the 2006 U.S. Open champion wouldn't win another point.
"I came back. I had chances. There's no doubt I had chances," Sharapova said. "But I guess today was the day I didn't take them."
Aside from all of those double-faults -- including two to begin the final game -- Sharapova finished with a total of 60 unforced errors, twice as high as her winner count.
"I didn't feel comfortable with most of my game today," Sharapova said.
Because of her Grand Slam pedigree and recent play, she was seen as someone who'd stick around deep into the second week at Flushing Meadows. Instead, Sharapova joined the reigning Wimbledon (Petra Kvitova) and French Open (Li Na) champions in leaving quickly, while two-time defending U.S. Open champion Kim Clijsters didn't enter because of injury.
Ninth-seeded Sam Stosur edged No. 24 Nadia Petrova 7-6 (5), 6-7 (5), 7-5 to reach the fourth round of the U.S. Open, barely avoiding the tournament's first 39-game, three-tiebreaker women's match in 20 years.
Stosur, the 2010 French Open runner-up, converted her fifth match point, breaking serve when Petrova netted a forehand after 3 hours, 16 minutes of play.
Had Petrova held serve there, they would have headed to a tiebreaker to decide the outcome. The other Grand Slam tournaments don't use tiebreakers in the third set of women's matches or the fifth set of men's.
Since the tiebreaker system began in 1970, only two U.S. Open women's singles matches had three of them: victories by Gigi Fernandez in 1991, and by Steffi Graf in 1985.
In other women's third-round matches, 2010 Wimbledon and U.S. Open runner-up Vera Zvonareva beat No. 30 Anabel Medina Garrigues 6-4, 7-5, and No. 13 Peng Shuai defeated No. 19 Julia Goerges 6-4, 7-6 (1).
Peng will take on Pennetta, a doubles champion at the Australian Open in January and a two-time U.S. Open quarterfinalist who covered every inch of the court and kept extending points against Sharapova.
"She played really smart," Sharapova said. "That's her game, and she makes you hit a lot of balls."
Pennetta was unaware of Sharapova's 12-0 mark in third sets entering the day (the Italian was 9-7). But she did know she could count on Sharapova to miss her share of serves. When a reporter began to ask about Sharapova's late double-faults, Pennetta interrupted to point out, "I was happy."
Ever since returning to the tour after October 2008 surgery on her right, racket-swinging shoulder, former No. 1 Sharapova has worked to reshape her serve and return to the top of tennis. And she did appear to be back to -- or at least near -- her best in recent months, winning titles at Rome on clay and Cincinnati on hard courts, reaching the semifinals at the French Open and the final at Wimbledon.
But a pattern developed, too: In her loss at Roland Garros, for example, she hit 10 double-faults, including on match point. In her semifinal victory at the All England Club, she overcame 13 more.
So Pennetta explained she wanted to be "really aggressive" on returns "to just let her think too much and maybe make some double-faults. That's what happened."
The Associated Press contributed to thsi report.