World No. 1 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 No. 1 Ana Ivanovic and number seven Mario Bartoli of France in previous rounds.
Russian World No. 2 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 No. 1) but I feel good and confident being there," she said.
Photograph: Clive Brunskill/Getty Images