Serena withdraws from Dubai

Defending champion Agnieszka Radwanska led several of the top seeds into the Dubai Championships quarterfinals on a Wednesday overshadowed by the withdrawal of Serena Williams because of a back injury.

Williams, who became the oldest top-ranked woman Monday, said she's had a sore back and ankle since the Australian Open, though she still reached the final in Qatar last weekend.

Her departure, announced 45 minutes before she was to face 11th-ranked Marion Bartoli, came a day after second-ranked Victoria Azarenka, who beat Williams in the Doha final, withdrew because of a right foot bone bruise.

''I've just had some back problems the past couple weeks. It didn't get better,'' Williams said. ''I thought it would get better as the week went on, but it didn't. I don't want to keep pushing it and make it worse.''

Former No. 1 Caroline Wozniacki joined Radwanska in the quarters, brushing aside an error-prone Zheng Jie of China 6-1, 6-0. Former US Open champion Sam Stosur beat Hseih Su-wei 6-4, 6-0.

Former Wimbledon champion Petra Kvitova also got past former No. 1 Ana Ivanovic 7-5, 7-6 (1).

The only upset was fourth-seeded Angelique Kerber, who racked up 10 double faults in falling to Roberta Vinci of Italy 5-7, 6-1.

Radwanska had a tougher-than-expected test in winning 7-5, 6-3 against Yulia Putintseva of Kazakhstan, who has only beaten two top 50 players.

Slow to adjust to slower court conditions from the previous week in Doha, the fourth-ranked Radwanska was down a break in both sets. After a five-minute rain delay in the first set, the Pole broke back to make it 4-4 and then won the set when Putintseva missed easy shots.

The 101st-ranked Putintseva broke to go up 2-1 in the second, but Radwanska broke right back and later won three straight games to finish off the match.

''Even when I didn't play my best tennis today, I was really fighting for every point and just happy, especially about the first set, that I could come back and then win that match in two sets,'' Radwanska said.

After a 2012 marked by injuries and disappointment at the Grand Slams, Wozniacki said her confidence was back. It has shown in the past two weeks, where she lost a close match to Radwanska and has only lost five games this week.

''I'm making the right decisions on court in terms of where I play the ball and, you know, not force myself to do things that aren't necessary,'' said Wozniacki, who was back in the top 10. ''That's mostly it, you know, the understanding of the game. And just enjoying being out there.''

Kvitova struggled for much of her match with her serve - finding the mark with only 57 percent of her first serves and committing eight double faults, one of which cost her a game when she was serving for the first set.

''It started pretty well. Then Ana tried to play quite aggressively, and then I had some trouble,'' the eighth-ranked Kvitova said. ''So she came back. She served quite well.''

Bartoli received a walkover and will face Wozniacki, the 2011 champion, in the quarterfinals.