DUBAI, United Arab Emirates — Defending champion Agnieszka Radwanska led several of the top seeds into the Dubai Championships quarterfinals on a day 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 withdrew because of a right foot bone bruise.
Former No. 1 Caroline Wozniacki joined Radwanska in the quarters, brushing aside an error-prone Zheng Jie of China 6-1, 6-0. Former U.S. 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.