After officially returning to the top of the women's rankings this week, Serena Williams decided to pull out of the Dubai Tennis Championships because of a lower back injury.

The reigning Wimbledon, U.S. Open and Olympic champion actually took over the No. 1 ranking last week in Qatar, marking the sixth time in her career she has been the top-ranked woman in the world. In doing so, the 31-year-old Williams became the oldest-ever No. 1.

A second-seeded Williams was scheduled to meet former Wimbledon runner-up Marion Bartoli on Wednesday. The wild card Bartoli, instead, received a walkover into the quarterfinals.

Former No. 1 and 2010 runner-up Victoria Azarenka pulled out of Dubai on Tuesday, citing a foot injury. The Australian Open titlist beat Williams in last week's marquee 1-versus-2 final in Doha.

On the court Wednesday, third-seeded reigning Dubai champion and 2012 Wimbledon runner-up Agnieszka Radwanska handled promising 18-year-old Kazakhstani wild card Yulia Putintseva 7-5, 6-3 and sixth-seeded former Wimbledon champ Petra Kvitova doused former world No. 1 Ana Ivanovic 7-5, 7-6 (7-1).

Radwanska beat German Julia Goerges in last year's Dubai finale.

A Day-3 upset came when Italian Roberta Vinci took our fourth-seeded German left-hander Angelique Kerber 7-5, 6-1.

In other second-round matches involving seeds, No. 5 French Open runner-up Sara Errani of Italy dismissed Romanian Sorana Cirstea 6-4, 6-4, No. 7 former U.S. Open champion Samantha Stosur whipped Taiwan's Su-Wei Hsieh 6-4, 6-0 and No. 8 former top-ranked star Caroline Wozniacki dismantled Chinese qualifier Zheng Jie 6-0, 6-1.

Also, Russian Nadia Petrova leveled Spanish lucky-loser Carla Suarez Navarro 6-1, 6-0. Suarez Navarro replaced Azarenka in the field.

Friday's quarterfinals will pit Radwanska against Kvitova, Errani versus Petrova, Stosur against Vinci, and the 2011 Dubai runner-up Wozniacki versus Bartoli.