BERLIN - Top-ranked Victoria Azarenka had to exert every ounce of her energy to beat German wild card Mona Barthel 6-4, 6-7 (3-7), 7-5 in a match that lasted for nearly three hours at the Porsche Grand Prix in Stuttgart, Germany on Friday.

With Azarenka, Maria Sharapova, Petra Kvitova and Agnieszka Radwanska all in Saturday's semi-finals, this is the first time since the Wimbledon Open in 2009 that the world's top four players have reached the last four at a WTA tournament.

Second-ranked Sharapova rallied to outlast No. 5 Sam Stosur of Australia 6-7 (5-7), 7-6 (7-5), 7-5 in a match that went one minute over three hours.

Fourth-seeded Radwanska rallied past No. 8 Li Na of China 3-6, 6-2, 6-3, while third-seed Kvitova advanced with a 6-4, 6-4 win over Germany's Angelique Kerber.