Top-ranked Victoria Azarenka needed nearly 3 hours to overcome big-serving Mona Barthel 6-4, 6-7 (3), 7-5 on Friday for a place in the Porsche Grand Prix semifinals that feature the top four players in the world rankings.

Azarenka will face fourth-ranked Agnieszka Radwanska, a 3-6, 6-2, 6-3 winner over Li Na.

Second-ranked Maria Sharapova rallied to outlast Sam Stosur of Australia 6-7 (5), 7-6 (5), 7-5 in a match that went a minute over 3 hours. Sharapova will face third-ranked Petra Kvitova, the Wimbledon champion who beat Angelique Kerber 6-4, 6-4 in the last quarterfinal.

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-high fifth title of the year.