Defending champion Agnieszka Radwanska and Australia's Samantha Stosur led a parade of top seeds into the Dubai Open quarter-finals on a Wednesday overshadowed by the abrupt withdrawal of Serena Williams with a back injury.
Williams, who became the oldest top-ranked woman on Monday, said she's had a sore back and ankle since the Australian Open, though she still reached the final in Doha last weekend.
Her departure, announced 45 minutes before she was due to face 11th-ranked Marion Bartoli, came a day after second-ranked Victoria Azarenka, who beat Williams in the Doha final, withdrew with 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 Stosur got the better of Hseih Su-wei 6-4 6-0.
Former Wimbledon champion Petra Kvitova also got past former No.1 Ana Ivanovic 7-5 7-6 (7-1).
The only upset on the day was fourth-seeded Angelique Kerber, who racked up 10 double faults in falling to Roberta Vinci of Italy 5-7 6-1. It was the latest disappointment for the German, who crashed out of the Qatar Open in the round of 32.
Radwanska had a tougher-than-expected test in winning 7-5 6-3 against 18-year-old wild card Yulia Putintseva of Kazakhstan.