Ana Ivanovic of Serbia reacts as she leaves after withdrawing in her quarter-final match against Agnieszka Radwanska of Poland in the China Open tennis tournament at the National Tennis Center in Beijing, Oct 7, 2011. [Photo/Agencies]

BEIJING - Former world number one Ana Ivanovic was the latest high-profile player to exit the China Open on Friday when the Serb retired from her quarter-final against Agnieszka Radwanska with back problems.

The 23-year-old was a set down when the injury she sustained last week in Tokyo flared up in the second set.

Maria Sharapova and Serena Williams pulled out of the competition a week ago, Victoria Azarenka failed to show for her match on Thursday due to injury while home hope Li Na exited in the first round, Sam Stosur went out in the second and Vera Zvonareva the third.

In the men's draw, world No 1 and 2010 champion Novak Djokovic was forced to pull out because of a back injury.

Caroline Wozniacki is the only top 10 women's player left in the event and the Dane is playing Flavia Pennetta on Friday.

Ivanovic said she had been fighting a losing battle all week.

"I have had lots of treatments on the injury, but it hasn't got much better," said the Serb. "I've been fighting it the whole week. I had an MRI here yesterday. The doctors are still looking at the results.

"I'm talking to physios and just making sure about the next steps to see what's wrong and what's going on. It's too early to say anything."

Romanian Monica Niculescu came from behind to beat Maria Kirilenko 3-6 6-3 6-4 to clinch a semi-final berth while in the men's competition Tomas Berdych thrashed Spaniard Fernando Verdasco 6-1 6-0 to seal a place in the last four.

The Czech faces the winner of the evening clash between Frenchman Jo-Wilfried Tsonga and Spain's Juan Carlos Ferrero.