| Updated at: 0437 PST, Friday, April 22, 2011|
STUTTGART, Germany: World number one Caroline Wozniacki avenged her shock Miami defeat to Andrea Petkovic by battling back to beat the German crowd favourite 6-4, 6-1 and reach the Stuttgart semi-finals on Thursday.
Despite finding herself 4-1 down after just 21 minutes of the first set, the 20-year-old Danish top seed roared back to break Petkovic, ranked 19th in the world, three times to take the opener.
Petkovic then ran out of steam and the 23-year-old had few answers as Wozniacki blazed into a 5-0 lead in the second set.
Wozniacki will now play Poland's Agnieszka Radwanska in Saturday's semi-final after the 22-year-old needed two hours to beat German wild-card Kristina Barrois 7-5, 6-3.
Earlier, Australia's Samantha Stosur recovered from losing the first set to knock out second-seed Vera Zvonareva.
The 27-year-old fifth-seed battled back for a 2-6, 6-3, 7-6 (7/3) victory over the Russian, who is ranked third in the world, in just over two-and-a-half hours.
Having made an early exit from January's Australian Open, Stosur, ranked seventh in the world, is hoping to use Stuttgart to recapture the form which helped her reach the French Open final last year.
In her semi-final, Stosur will play Germany's Julia Goerges, ranked 32nd in the world, who beat the Australian's doubles partner Sabine Lisicki 6-4, 6-4.
The German is still in the hunt to claim the winners prize of a brand-new sports car worth 111,000 US dollars.