Samantha Stosur avenged last year's semi-final defeat by Julia Goerges, beating the German 6-2 2-6 6-3 at the WTA event in Stuggart.
The Australian number five's win sets up a quarter-final against world number two Maria Sharapova.
World number one Victoria Azarenka also moved into the quarters after Germany's Andrea Petkovic retired with an ankle injury while trailing 6-2, 4-4.
Azarenka will face in-form Mona Barthel of Germany for a spot in the last four.
Petkovic looked set to give her opponent a tougher time but badly twisted her ankle as she tried to hit a forehand.
"It's such a tough situation and the ankle right away blew up," said Azarenka in a courtside interview.
"I hope she recovers soon because she is a great player. Andrea is one if the players who is tough mentally, she is a great fighter," she said.
German Barthel kept up her spectacular form this year with a 6-3, 6-1 demolition of world number seven Marion Bartoli to book her spot in the last eight and notch her first career victory over a top-10 player.
"It was one of the best matches of my life," said Barthel, who won her maiden WTA title in Hobart in January.
"I was immediately able to put her serve under pressure. It's great to have won my first match against a Top 10 player. It's something special."
Wimbledon champion Petra Kvitova also advanced with a comfortable straight-sets victory over Italy's Francesca Schiavone and China's Li Na beat Akgul Amanmuradova of Uzbekistan 6-4 6-4.
ReutersTags: tennis, sport, germany First posted April 27, 2012 05:53:25