Flavia Pennetta of Italy returns the ball to Caroline Wozniacki of Denmark during their quarter-final match in the China Open tennis tournament at the National Tennis Center in Beijing, Oct 7, 2011. [Photo/Agencies]

BEIJING - Italian veteran Flavia Pennetta on Friday scored a gritty victory over world number one and defending champion Caroline Wozniacki of Denmark to set up a semifinal clash against Agnieszka Radwanska of Poland at the China Open.

Wozniacki took the first set routinely but faded off in the last two sets, slumping at 3-6, 6-0, 7-6 (2) to the 26th ranked Pennetta in two hours and 14 minutes.

Radwanska, seeded 11th in the elite tournament, had a much easier time of it in the quarterfinals when Serb Ana Ivanovic pulled out while trailing 6-3, 3-2, citing a back injury. The former French Open champion also retired from the Beijing Olympics three years ago on the same court.

"I love coming here and competing," said Ivanovic. "It was very tough to pull out of Olympics, and it's very tough today."

Ivanovic added she had been struggling with her back since the beginning of last week in Tokyo, and now "it hasn't been really getting much better".

The 23-year-old called two medical timeout before and during the second set of the quarterfinal, but it apparently did no good to her injury.

Radwanska, who has just been crowned at the Pan Pacific Open and thus enjoyed first round bye at the China Open, should feel comfortable after the easy match while Pennetta, making both singles and doubles semifinals, needs careful prep work.

Flavia Pennetta of Italy reacts as she celebrates after defeating Caroline Wozniacki of Denmark during their quarter-final match in the China Open tennis tournament at the National Tennis Center in Beijing, Oct 7, 2011. [Photo/Agencies]

Wozniacki, overall a defesive baseliner, dropped serve twice but broke back three times in the first set, but it was the end of her good fortunes at this year's China Open.

The 29-year-old Pennetta taught the world number one a good lesson on how to score from the baseline by hitting efficient and angled groundstrokes in the remaining time, claiming the second set at love and then storming to a 4-1 lead in the third.

Wozniacki, eight years Pennetta's junior, clawed back into contention by tying the final set at 6-all, but the Dane blew out the confidence she built earlier with a 7-2 loss in the tiebreak.

"It was a tough match and we had a lot of good relies. It's disappointing to lose, especially when you have your chances like I had," Wozniacki said after the match.

Wozniacki's knockout was just one of the many surprises that kept on coming at the $4.5 million tournament, which was marred by a string of pull-outs and upsets.

Earlier in the day, Romanian qualifier Monica Niculescu continued her career best run at the China Open with another impressive comeback win against higher ranked Russian Maria Kirilenko 3-6, 6-3, 6-4.

The 24-year-old Romanian qualifier had beaten No 4 seed and French Open champion Li Na earlier in the week and now she has reached her third and biggest WTA semifinal of her career.

In the semifinals Niculescu will face Germany's in-form star and ninth seed Andrea Petkovic, who got the better of 13th seed Anastasia Pavlyuchenkova from Russia 6-3, 7-6 (2).

In the ATP event, third seed Tomas Berdych of the Czech Republic swept past Spaniard Fernando Verdasco 6-1, 6-0 for a 6-1 lifetime record and also set up a semifinal clash against top seed Jo- Wilfried Tsonga from France, who defeated Spain's former world number one Juan Carlos Ferrero.

Croat Ivan Ljubicic needed almost three hours to dispatch Russian Mikhail Youzhny 7-6 (4), 6-7 (4), 6-2 and will in the other semifinal meet compatriot Marin Cilic, who overcame Kevin Anderson of South Africa 7-6(4), 4-6, 6-1.