WTA China Open: Ana Ivanovic into semi-finals after Simona Halep withdraws through injury

Ana Ivanovic was handed a free passage through to the semi-finals of the China Open after Simona Halep was forced to withdraw from the tournament with a left hip injury.

The second-seeded Halep had secured a gruelling 7-6 (7-4) 5-7 7-6 (7-1) victory against Germany's Andrea Petkovic before pulling out of Friday's meeting in Beijing against the Serb.

I'm happy I was able to win and finish the match in the right way, but I have a hip injury and it would be risky for me to play again tomorrow.

Halep took a medical timeout for treatment on her hip after the first set, but returned to complete the match, which featured 17 breaks of serve and 105 combined unforced errors (61 from the Romanian).

"It was a tough match against Andrea today, and I had to fight a lot," said the French Open runner-up. "I'm happy I was able to win and finish the match in the right way, but I have a hip injury and it would be risky for me to play again tomorrow."

Halep's withdrawal gave ninth-seeded Ivanovic, who is fighting for a spot in the WTA finals in Singapore, a last four spot after she beat big-hitting Sabine Lisicki 6-3 7-5.

Serena battles through

Two-time Wimbledon champion Petra Kvitova also advanced when Venus Williams withdrew before their match with a viral illness.

Venus' sister, Serena, retired from a match last week at the Wuhan Open, citing a virus, as well, but the defending champion was back to winning ways after a hard-fought 6-1 1-6 6-2 victory against gutsy Czech Lucie Safarova to reach the quarter-finals.

Williams fired down 11 aces but also reeled off six double faults on her way to victory over the 27-year-old in one and a half hours.

Two other seeded players were knocked out of the tournament, shaking up the race for the WTA finals in Singapore.

Seventh-seeded Angelique Kerber was defeated by two-time major winner Svetlana Kuznetsova 2-6 6-4 6-3, while 12th-seeded Ekaterina Makarova fell to Roberta Vinci 6-1 0-6 7-5.

Because of those losses, Wimbledon finalist Eugenie Bouchard and Agnieszka Radwanska, both of whom lost early in Beijing, are assured of making the end of season field in Singapore.

Australia's Sam Stosur ended Alize Cornet's interest in the tournament with a routine 6-4 6-2 win over the Frenchwoman.