Simona Halep, Petra Kvitova through to China Open quarters

Romania's Simona Halep won a back-and-forth match against Andrea Petkovic featuring 17 breaks of serve, beating the German 7-6 (4), 5-7, 7-6 (1) to advance to the China Open quarter-finals on Thursday.

The second-seeded Halep was broken nine times, including when she served for the match at 5-3 in the third set, but took the tiebreaker comfortably to close out the win.

Halep, this year's French Open finalist, next plays ninth-seeded Ana Ivanovic of Serbia, a 6-3, 7-5 winner over Sabine Lisicki of Germany.

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

Venus' sister, Serena, withdrew from a tournament last week in the central Chinese city of Wuhan citing a virus, as well. Serena Williams, the defending champion, plays Lucie Safarova for a spot in the quarters later Thursday.

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

Seventh-seeded Angelique Kerber of Germany was defeated by two-time major winner Svetlana Kuznetsova 2-6, 6-4, 6-3, while No. 12-seeded Ekaterina Makarova of Russia fell to Italy's Roberta Vinci 6-1, 0-6, 7-5.

Because of those losses, Eugenie Bouchard of Canada and Agnieszka Radwanska of Poland, both of whom lost early in Beijing, are assured of making the eight-woman field in Singapore. Kerber's chances of qualifying, however, now appear slim.

On the men's side, third-seeded Tomas Berdych stalled Viktor Troicki's comeback from a year-long suspension for a missed drug test with a 6-3, 6-4 victory.

Troicki, whose ranking peaked at No. 12 in 2011, was playing in just his third ATP-level tournament since returning from his ban in July for skipping a blood test in April 2013. His initial 18-month ban was reduced to a year on appeal.

Seventh-seeded Ernests Gulbis of Latvia retired with a shoulder injury while trailing Slovakian qualifier Martin Klizan 6-2, 3-0.

Second-seeded Rafael Nadal and No. 6 Andy Murray played their third-round matches later Thursday.