Zvonareva defeated Vera Dushevina 6-3, 6-0 in 61 minutes at the La Costa Resort and Spa. She won her first championship of the year two weeks ago in Azerbaijan before heading to the United States for a series of events leading up to the U.S. Open.
Second-seeded Andrea Petkovic, who has won two tournaments this year, made it to the quarters when Virginie Razzano retired in the second set. Petkovic had a 6-2, 3-0 lead when Razzano left the court with a right-shoulder injury.
Another withdrawal left 18-year-old American Shane Stephens heading for the first WTA Tour quarterfinal match of her career. Stephens advanced when Tamira Paszek forfeited the match at 5-3 in the first set due to an abdominal strain.
Stephens, who had never won more than one match in a tournament prior to this week, will take on Petkovic in the quarterfinals.
Also advancing were Ana Ivanovic, Agnieszka Radwanska and Peng Shuai. Radwanska lost in the finals of this tournament a year ago to Svetlana Kuznetsova.
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