The Russian world tennis number two Maria Sharapova needed three hours to get past Australia's Sam Stosur 6-7 7-6 7-5 to reach the Stuttgart indoor tournament semi-finals, where she'll take on Wimbledon champion Petra Kvitova.
The third-seeded Czech Kvitova enjoyed a 6-4 6-4 win over Germany's Angelique Kerber, while the world number one Victoria Azarenka had to dig deep to overcome the hugely talented Mona Barthel of Germany 6-4 6-7 7-5.
Azarenka, the Australian Open champion, now faces Poland's Agnieszka Radwanska, after the fourth seed recovered from a poor start to beat China's Li Na 3-6 6-2 6-3.
