World number one Victoria Azarenka had to dig deep to overcome the hugely talented Mona Barthel of Germany 6-4 6-7 7-5 to reach the Stuttgart indoor tournament semi-finals on Friday.
The Australian Open champion, who now faces Poland's Agnieszka Radwanska, won a hard-fought first set but had to fight off a sustained challenge from Barthel.
The German eliminated former world number one Ana Ivanovic and number seven Mario Bartoli of France in previous rounds.
Russian world number two Maria Sharapova needed three hours to get past Australia's Sam Stosur 6-7 7-6 7-5 and will now take on Wimbledon champion Petra Kvitova after the third-seeded Czech enjoyed a 6-4 6-4 win over Germany's Angelique Kerber.
Earlier, fourth seed Radwanska recovered from a poor start to beat China's Li Na 3-6 6-2 6-3.
"She showed some great tennis today," Azarenka said of Barthel in a courtside interview. "I always said since the start of the year she is the one to watch."
Belarussian Azarenka has lost only once this season after going 26 matches without defeat.
"I am trying to become better and better. This is new for me (being number one) but I feel good and confident being there," she said.
The pair traded punishing blows in the second set with Barthel twice moving within two points of winning it, at 5-4 and 6-5, before Azarenka briefly got herself out of trouble.
Big-serving world number 35 Barthel, who won her maiden WTA tour event in Hobart in January, was fired up by a local crowd mesmerised by her mix of raw power and intelligent play. That helped the German grab the second set on her first set point.
A huge backhand winner down the line brought up the first break opportunity at the start of the third set and Azarenka cracked under pressure to send the next point long.
But she recovered to twice break back and, after squandering a match point at 5-4, finished her opponent off two games later.
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