Top-ranked Victoria Azarenka needed nearly three hours to overcome big-serving wild card Mona Barthel 6-4, 6-7 (3), 7-5 for a place in the Porsche Grand Prix semifinals on Friday.
Second-ranked Maria Sharapova was on court for an even longer period, rallying to outlast No. 5 Sam Stosur of Australia 6-7 (5), 7-6 (5), 7-5 in a match that went one minute over three hours.
Fourth-seeded Agnieszka Radwanska rallied past No. 8 Li Na 3-6, 6-2, 6-3. Wimbledon champion and third-ranked Petra Kvitova defeated Angelique Kerber 6-4, 6-4 in the last quarterfinal to make sure the four top-ranked players are all in the semifinals of a tournament for the first time since Wimbledon 2009.
In the semis, Azarenka plays Radwanska and Kvitova goes against Sharapova.
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 two 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.
Stosur, the U.S. Open champion, played nearly flawless tennis and a double-fault by Sharapova 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. She is now 10-1 against Stosur.
