Peng Shuai of China hits a return to Flavia Pennetta of Italy during their match at the US Open tennis tournament in New York, Sept 4, 2011. [Photo/Agencies]

NEW YORK - Italy's Flavia Pennetta felt sick to her stomach on a hot, muggy day at the US Open Sunday, but dug deep to push on and beat 13th-seeded Peng Shuai of China 6-4 7-6 and reach the quarter-finals.

Pennetta persevered through a spell of dry retching when she was two points away from ending the match in the 12th game, and then fought back from 6-2 down in the second-set tiebreaker by winning the last six points to take the decider 8-6 and end the two hour 31 minute struggle.

It was the second successive upset registered by the 29-year-old Italian, who toppled third-seeded former champion Maria Sharapova in a third-round shocker.

"I was feeling really bad," said Pennetta, who became physically ill behind the baseline serving for the match with a 30-love advantage.

"I think it was because it's really humid today. It's hot. And also, when you are there, you have a lot of emotion in the court.

"My body just need to breathe, and I starting maybe to have the sensation to throw up. But doesn't, without nothing inside, so it didn't come out."

Peng, 25, saved one match point in the 12th game before breaking serve to send the set to a tie-break, and raced to a 5-1 then 6-2 lead against her struggling opponent.

However, Pennetta revived and clawed her way back. "I just believe all the time, and I just try to play every point. Was working pretty good," she said.

Pennetta has had her best grand slam successes in New York, reaching the quarters in three of the last four years, losing to then number one Dinara Safina and three-time champion Serena Williams in the other.

This time, 26th-seeded Pennetta will play 92nd-ranked Angelique Kerber of Germany, a straight-set winner over Romanian Monica Niculescu.

Flavia Pennetta of Italy hits a return to Peng Shuai of China during their match at the US Open tennis tournament in New York, Sept 4, 2011. [Photo/Agencies]

Pennetta said she would not underestimate Kerber, 23, as she aims for her first grand slam semi-final. "She's in the quarter-finals. You are not there because you want to be there. You are there because you beat a lot of good players," she said.

Pennetta said she was also not worried about being one of the oldest players in the women's draw.

"I know I'm a little bit old for tennis. For life I'm young," she said, adding that mature players have had recent success in the majors.

"Actually we have a good experience, like Francesca (Schiavone of Italy) last year, she won Roland Garros and she was 30, and this year the final at 31. Li Na, she won Roland Garros and she's 29, also.

"There are some players that can grow up early, and some players they need time. I'm one of them."