World number one Caroline Wozniacki survived an early scare to win 10 straight games and crush German Andrea Petkovic 6-4, 6-1 on Thursday for a spot in the last four of the Stuttgart Grand Prix.
Wozniacki, already a winner of three titles this year, will next face Poland's Agnieszka Radwanska who beat German Kristina Barrois 7-5, 6-3 on the clay.
Australian Samantha Stosur also booked her spot with a gutsy 2-6, 6-3, 7-6 win over world number three Vera Zvonareva, who broke a staggering nine strings and briefly played with her coach's racquet.
World number 19 Petkovic, who has compared herself to a diesel engine because of her slow starts in previous rounds, shot out of the blocks and twice broke Wozniacki to go 4-1 up.
Wozniacki hung on, saving four break points to avoid falling 5-1 behind.
The Dane, eyeing the French Open next month as she seeks her maiden grand slam title, then played more aggressively by coming to the net and snatched the set before breezing through the second against the frazzled German.
"Definitely it was a fight back to win that first set," Wozniacki said in a courtside interview.
"I am sorry to beat the home favourite but am happy to be in the semis and hope you will support me," she told the crowd.
"Today I felt good in here and my volleys felt great which I have been working on and they normally don't. I am pleased at the way I was able to finish the match today," she added.
Stosur, last year's French Open finalist, skipped the Fed Cup playoff against Ukraine at the weekend to focus on her clay court preparation and it paid off as she fought back from a set down to beat the Russian.
Stosur will now face 22-year-old Julia Goerges, ranked 32nd, who beat fellow German Sabine Lisicki 6-4, 6-4.
"It is the best match I have had all year," Stosur said in a courtside interview.
"To come through as close and as tight is just wonderful," she added.
An opening game lasting 14 minutes set the tone for a tight encounter with second-seed Zvonareva breaking her opponent on her ninth opportunity.
Stosur initially looked uncomfortable on Stuttgart's indoor red clay in an error-strewn first set performance.
The 27-year-old battled her way back into the contest, serving with much more kick and hitting a better forehand to lead 5-3 and grab the second set when Zvonareva fired a service return long.
The Russian seemed not to be distracted by the number of times she had to switch racquets even when she broke strings on two consecutive points.
The pair matched blows before Stosur played a near flawless tiebreak to clinch the match on her first match point.
In an all-German hard-hitting battle, Goerges outgunned Lisicki to book her spot in impressive style.
The pair traded bludgeoning blows from the start and Lisicki, ranked 155th and getting back to her best after an injury-plagued 2010, recovered despite being a set and a break down to lead 4-3.
But Goerges fired two forehand winners to clinch another break and go 5-4 up before serving out the match.