STUTTGART, Germany -- World No. 1 Caroline Wozniacki was an easy quarterfinal winner, while second-seeded Vera Zvonareva was knocked out of the $721,000 Porsche Tennis Grand Prix.

The Danish Wozniacki handled German Andrea Petkovic 6-4, 6-1, while the 2010 U.S. Open and Wimbledon runner-up Zvonareva was dismissed by fifth- seeded Aussie slugger Samantha Stosur 2-6, 6-3, 7-6 (7-3) on the indoor red clay at Porsche Arena. Stosur has now beaten Zvonareva in their last six meetings, after the Russian captured their first two career WTA matchups.

Stosur was last year's Stuttgart runner-up to since-retired former world No. 1 great Justine Henin.

In an all-German second-round affair on Day 4, Julia Goerges topped wild card Sabine Lisicki 6-4, 6-4. Goerges and Lisicki were part of a German Fed Cup team that flattened the United States 5-0 in a World Group Playoff on this same court this past weekend.

Goerges should have her hands full with Stosur in Friday's semifinals.

The 2011 Porsche champion will collect $111,000.