Top-ranked Caroline Wozniacki reeled off 10 straight games to rally for a 6-4, 6-1 victory over Andrea Petkovic on Thursday and a place in the Porsche Grand Prix semifinals.
Wozniacki was 4-1 down and one point from falling 5-1 behind before she broke Petkovic's serve. She also saved four break points in the next game to get within 4-3.
Petkovic could not hold serve in the next game either and the No. 1 from Denmark rolled on in her bid for her fourth title of the year.
Wozniacki next plays Agnieszka Radwanska of Poland, who beat Germany's Kristina Barrois 7-5, 6-3.
Sam Stosur also rallied, advancing with a 2-6, 6-3, 7-6 (3) victory over second-seeded Vera Zvonareva.
Stosur, the Porsche runner-up last year, extended her winning streak against Australian Open semifinalist Zvonareva to six matches and reached her first semifinal this year.
Stosur next plays Julia Goerges, who edged fellow German Sabine Lisicki 6-4, 6-4. Goerges trailed 4-3 in the first set, but won five straight games to take control of the match.
''I knew she is a good player on a comeback. I had to play my game,'' said Goerges, the first German semifinalist in Stuttgart since Anke Huber in 1999.
Lisicki was once ranked No. 22, but had a long-term ankle injury and entered the tournament as a wild card ranked 155th.
Stosur, runner-up at last year's French Open, has struggled this year and was playing only her second quarterfinal of the year, despite being ranked No. 7 in the world.
''This is the best match I had all year. She played well and I had to hang in there,'' Stosur said.
Zvonareva, the champion in Doha in February, received treatment on her right shoulder in the second set.
Wozniacki lost to Petkovic in Miami in three sets on a hard court, but was more comfortable on the indoor clay in the Porsche Arena.
''I changed my tactics, it was a different surface and even my volleys were working great. From 1-4 on I played some great tennis,'' said Wozniacki, an Australian Open semifinalist. ''Andrea is a great player and it was good to come back and win the first set. I got off to a great start in the second.
''I just kept on playing. ... On a clay court you can always come back, no matter what the score is.''
Petkovic was looking for a repeat of her week in Miami, where she beat Wozniacki and former No. 1 Jelena Jankovic. But after besting Jankovic again in the second round Wednesday, Petkovic ran out of steam against Wozniacki.
To illustrate how tired she was, Petkovic jokingly arrived at the news conference in a wheelchair pushed by her coach.
''I had a lot of chances, but I didn't have the legs to use them,'' Petkovic said. ''You have to be able to run for three hours when you play Caroline, every point is hard work. I had more energy in Miami and I was able to run her down there. She raised her game noticeably at 4-1, she became more aggressive.''
Petkovic, who helped Germany beat the U.S. 5-0 in Fed Cup last weekend, has led a revival of women's tennis in Germany. She jumped to a career-high No. 19 ranking this month and now is likely to break into the top 15.
