US Open winner Samantha Stosur will face world number two Maria Sharapova in Friday's quarter-finals at Stuttgart's WTA tournament after beating defending champion Julia Goerges.
In a repeat of last year's semi-final, Stosur, 28, was taken to three sets by Germany's Goerges in Thursday's second-round clash before the Australian dug deep for a 6-2, 2-6, 6-3 victory at the claycourt tournament.
Wild-card Mona Barthel claimed her second top scalp of the week to also reach the last eight as she knocked out seventh seed Marion Bartoli.
The 21-year-old German, who blasted down 11 aces to out-muscle ex-world number one Ana Ivanovic on Wednesday in straight sets, needed just 71 minutes to beat France's Bartoli 6-3, 6-1 as she hit a further seven aces.
In Friday's last eight, Barthel, ranked 35th in the world, will now face either current world number one Victoria Azarenka or Germany's Andrea Petkovic, who meet later on Thursday.
"My opponent had nothing to lose, that makes a first time head-to-head difficult," said Bartoli of Barthel, who is part of a batch of promising German female tennis players having won the Hobart WTA tournament in January.
"Mona played a lot tougher than I thought. I had watched her playing Ana and had noticed that she has a hard serve and strong strokes.
"But I was still surprised about how fast she could play, I virtually had no chance to defend, especially on this slippery surface, at times it left like I was playing on ice.
"This court contributed to Mona's game as she is very tall and can play very fast, so it was very hard for me to break her rhythm. I am sure the match between us would look very different on a hard court."
There were no such problems for French Open winner Li Na, the eighth seed, who beat Akgul Amanmuradova of Uzbekistan 6-4, 6-4 and the Chinese will play Poland's Agnieszka Radwanska in the quarter-finals.
Wimbledon champion Petra Kvitova had few problems in her 6-2, 6-2 win over Italy's Francesca Schiavone to set up a last eight match against either ex-world number one Caroline Wozniacki or Germany's Angelique Kerber.