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 semifinals of the Porsche Grand Prix.
Wozniacki was 4-1 down and one point away from falling 5-1 behind but broke Petkovic's serve to cut the deficit. She also saved four break points in the next game to come to within 4-3.
Petkovic could not hold serve in the next game either and the No. 1 from Denmark rolled on.
Last year's runner-up Sam Stosur also rallied to beat second-seeded Vera Zvonareva 2-6, 6-3, 7-6 (3).
Stosur extended her winning streak against Australian Open semifinalist Zvonareva to six matches and reached her first semifinal of the year.
Stosur's opponent in the last four will be Julia Goerges, who edged fellow German Sabine Lisicki 6-4, 6-4. Goerges trailed 4-3 in the first set, then 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," Goerges said.
Germany now has a semifinalist in Stuttgart for the first time since Anke Huber in 1999.
Lisicki was once ranked No. 22 but then suffered 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," Wozniacki said. "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."
Wozniacki is looking for her fourth title of the year and was a semifinalist at the Australian Open.
Petkovic was looking for a repeat of her week in Miami, when she beat both Wozniacki and former No. 1 Jelena Jankovic. But after beating Jankovic again in the second round on Wednesday, Petkovic ran out of steam against Wozniacki.
Petkovic has led a revival of women's tennis in Germany and jumped to a career-high No. 19 ranking in the world earlier this month.