STUTTGART, Germany -- Top-ranked Victoria Azarenka needed nearly three hours to overcome big-serving wild card Mona Barthel 6-4, 6-7 (3), 7-5 on Friday for a place in the Porsche Grand Prix semifinals.
Azarenka will face fourth-seeded Agnieszka Radwanska, a 3-6, 6-2, 6-3 winner over No. 8 Li Na.
Complete results
Need the scores from any match played in today's tournaments? Results
Second-ranked Maria Sharapova rallied to outlast No. 5 Samantha Stosur of Australia 6-7 (5), 7-6 (5), 7-5 in a match that went a minute over three hours. Sharapova will face the winner of a late match between third-seeded Petra Kvitova and Angelique Kerber.
Stosur, the U.S. Open champion, played nearly flawless tennis, and a double fault by the Russian gave her a match point at 5-3 in the second set. But Sharapova saved it with a forehand winner and held serve.
The first break point Stosur faced in the match came in the next game and she netted a forehand to allow Sharapova to level the score.
Sharapova then started raising her game to win the tiebreaker and take control.
She broke serve for a 6-5 lead and closed out the match in the next game despite a double-fault.
Barthel fired 13 aces but her serve deserted her in the final game. The German failed to win a point and double-faulted on match point, one of her 16 double faults.
Barthel came back from a break down and saved an earlier match point in the 10th game of the third set. The match on indoor clay lasted 2 hours, 48 minutes.
"She is very tough to play," Azarenka said. "She's much more confident now and her serve is right up there with the best. We were playing over two hours and she was still serving aces."
Azarenka, the Australian Open champion, is looking for a tour-best fifth title of the year.
