Serena Williams out with back injury

DUBAI, United Arab Emirates, Feb. 20 (UPI) -- World No. 1 Serena Williams, citing a lower back injury, withdrew Wednesday from the WTA's Dubai Duty Free Tennis Championships.

Williams, as the No. 2 seed, had a first-round bye but pulled out of the tournament before play began Wednesday. That gave Marion Bartoli a walkover into the quarterfinals.

Victoria Azarenka, whom Williams replaced last Monday as the No. 1-ranked player, said a foot injury forced her to leave the Dubai tournament.

That leaves defending champion Agnieszka Radwanska as the highest seed in the tournament. She defeated Yulia Putintseva 7-5, 6-3 in her second-round match by going 6-for-6 in break-point opportunities.

Roberta Vinci ousted fourth-seeded Angelique Kerber 7-5, 6-1 with four breaks in a seesaw first set and two more in the second.

No. 8-seeded Caroline Wozniacki blitzed Zheng Jie 6-0, 6-1 in 54 minutes, losing only eight points in seven games serving. Nadia Petrova also had a lopsided win, beating Carla Suarez Navarro 6-1, 6-0 without facing a break point. Suarez Navarro had taken Azarenka's spot in the draw.

No. 5-seeded Sara Errani defeated Sorana Cirstea 6-4, 6-4, sixth-seeded Petra Kvitova edged Ana Ivanovic 7-5, 7-6 (7-1) and No. 7-seeded Samantha Stosur beat Hsieh Su-Wei 6-4, 6-0.

Thursday's quarterfinals have Errani going against Petrova and Stosur facing Vinci in the top of the draw while Radwanska plays Kvitova and Wozniacki meets Bartoli in the other matches.