SAN DIEGO (AFP) - The top five seeds at the WTA San Diego Open sailed into the quarter-finals on Thursday, with only China's number four Peng Shuai taken the three-set distance. Top-seeded Vera Zvonareva led the way as she hammered fellow Russian Vera Dushevina 6-3, 6-0. German second seed Andrea Petkovic needed a mere 43 minutes to advance after French opponent Virginie Razzano retired with a shoulder injury while trailing 6-2, 3-0. And third-seeded Agnieszka Radwanska persevered through shoulder pain caused by a nerve problem to crush American teenager Christina McHale 6-1, 6-0. Radwanska admitted: "I'm worried about the shoulder. I'll have to get it looked at. "It seems every time I come on the the hardcourt in summer it gives me trouble. It's tough to treat." But she said: "I played better than in my first match, everything was working." In the quarter-finals Radwanska will face eighth-seeded Slovakian Daniela Hantuchova, who beat China's Zheng Jie 6-2, 4-6, 6-4. Peng needed two hours, 39 minutes to defeat Italian 15th seed Sara Errani 4-6, 6-2, 7-6 (9/7). She now faces fifth-seeded Serb Ana Ivanovic, a former world number one who was buoyed by by her 6-1, 6-2 defeat of Italian Alberta Brianti. "There were a lot of close games, overall I played very well," said Ivanovic, the 2008 Roland Garros champion and world number 18, who recently hired Briton Nigel Sears as a coach. "I was patient. If you give her chances she can do damage. I stayed calm and executed my shots like I've been doing in practice." Zvonareva posted her sixth victory in nine matches against Dushevina and reached her ninth quarter-final of the season. "It was an improvement over my first match," said Zvonareva, a finalist at Wimbledon and the US Open in 2010. "I started slow and had some ups and downs. I was able to go for my shots and played well on the key points. I never gave her a chance to turn it around or get back into the game." Zvonareva next faces Wimbledon semi-finalist Sabine Lisicki of Germany, a 6-4, 6-3 winner over American Coco Vandeweghe. Petkovic, who arrived at La Costa six days early and has been enjoying the luxury resort with her sister, will next play American surprise packet Sloane Stephens after the 18-year-old advanced when Austria's Tamira Paszek retired with an abdominal strain while trailing 5-3. Petkovic is rationing her American events in the lead up to the August 29 start of the US Open to avoid any chance of burnout. "I had no expectations coming here," said the world number 11, who is playing her first tournament since Wimbledon. "I prefer staying at home until the last minute and practicing. It's a long summer trip over here for Europeans."