Italian Flavia Pennetta stunned world number one Caroline Wozniacki with a fierce 3-6, 6-0, 7-6 (7/2) comeback in the quarter-finals of the China Open on Friday. The Dane looked to be in control before Pennetta began a fightback, winning the second set to nil, and dominating a third set tie-break. Pennetta, the world number 26, had lost all five of their previous meetings. Wozniacki has claimed six WTA trophies this season but -- despite her world ranking -- is still searching for her first Grand Slam crown. "I did my best today, but it just wasn't good enough," said a disappointed Wozniacki. "I had chances and didn't take them." Poland's Agnieszka Radwanska, the 11th seed and a winner last week in Tokyo, reached the final four after Serbian Ana Ivanovic retired with a back injury while trailing 6-3, 3-2. "I've been struggling with my back since actually beginning of last week in Tokyo," the former world number one said. "I've been doing lots of treatments on it, but it hasn't been really getting much better. "I surprised myself that I got to this stage at all." Giant-killing Romanian qualifier Monica Niculescu fought back from a set down against former champion Maria Kirilenko to reach the semi-finals and keep her dream run alive. Niculescu, the world number 57, who knocked out French Open champion Li Na in the first round, recovered from a slow start to win 3-6, 6-3, 6-4. "I'm really confident. I played four rounds at the US Open. I had a little break afterwards, I'm feeling fit and expecting a lot from myself," she said. Niculescu will play the winner of the match between German ninth seed Andrea Petkovic and Russian 13th seed Anastasia Pavlyuchenkova. German ninth seed Andrea Petkovic completed the semi-final lineup with her defeat of Anastasia Pavlyuchenkova 6-3, 7-6 (7/2). In the men's draw, third seed Tomas Berdych crushed Fernando Verdasco 6-1, 6-0 in 65 minutes, one of the heaviest defeats of the Spaniard's career. Berdych opened with a loss of serve but then turned on the afterburners to win 12 straight games. "The hard court is much more suitable for my game style, that I can play more aggressive," said the Czech. "I can give him much more pressure, and he makes mistakes. That's the game what I want to play." Berdych has yet to win a title in 2011. Only he and Frenchman Gael Monfils from the top 10 are without a trophy this year. He will play his seventh semi-final of the year against French top seed Jo-Wilfried Tsonga, who dispatched Spain's Juan Carlos Ferrero 6-3, 6-4. Croatian Ivan Ljubicic beat Russian Mikhail Youzhny 7-6 (7/4), 6-7 (4/7), 6-2 and will Croatian Marin Cilic, a winner over South African Kevin Anderson 7-6 (7/4), 4-6, 6-1.