STUTTGART, Germany - World number one Victoria Azarenka moved into the Stuttgart indoor tournament quarter-finals on Thursday as sixth seed Caroline Wozniacki was upset by Angelique Kerber.

Azarenka will face in-form Mona Barthel of Germany for a place in the last four after Germany's Andrea Petkovic retired with an ankle injury while trailing the Belarussian top seed 6-2 4-4.

Azarenka grabbed a break in the opening game but missed an easy volley to hand it back in the fourth game to Petkovic, who had only this week returned to the Tour after almost four months out with a back injury.

She then proceeded to break the German again to take the first set 6-2.

The second set initially followed a similar pattern with Azarenka breaking in the first game courtesy of a stunning double-handed backhand down the line before being broken straight back.

World number 12 Petkovic looked set to give her opponent a tougher time but badly twisted her ankle as she tried to hit a forehand at 4-4 with the indoor crowd instantly falling deadly quiet.

"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.

Andrea Petkovic of?Germany?twists her ankle?during her match against Victoria Azarenka of Belarus during day four of the WTA Porsche Tennis Grand Prix at Porsche Arena on April 26, 2012 in Stuttgart, Germany.[Photo/CFP]

German Mona 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."

World number five Sam Stosur avenged last year's semi-final defeat by Julia Goerges, beating the German defending champion 6-2 2-6 6-3 to set up a quarter-final against world number two Maria Sharapova.

Caroline Wozniacki of Denmark reacts during her match against Angelique Kerber of Germany during day four of the WTA Porsche Tennis Grand Prix at Porsche Arena on April 26, 2012 in Stuttgart, Germany.[Photo/CFP]

German Kerber also caused a major upset, dismantling former world number one Wozniacki 6-1 6-2 to set up a mouth-watering clash with Wimbledon champion Petra Kvitova.

The Czech advanced with a comfortable straight-sets victory over Italy's Francesca Schiavone while China's Li Na beat Akgul Amanmuradova of Uzbekistan 6-4 6-4.