Stuttgart - World No 1 Victoria Azarenka moved into the Stuttgart indoor tournament quarter-finals on Thursday 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.
The Belarussian top seed grabbed a break with the opening game but missed an easy volley to hand a break back in the fourth game.
She then proceeded to break the German, back for her first WTA tournament after a back injury ruled her out for almost four months, 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.
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.
"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."
World number five Sam Stosur avenged last year's semi-final defeat by Julia Goerges, beating the German 6-2 2-6 6-3 to set up a quarter-final against world number two Maria Sharapova.
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.
Results on Thursday from the Porsche Tennis Grand Prix on indoor clay at the Porsche Arena (seedings in parentheses):
Mona Barthel, Germany, def. Marion Bartoli (7), France, 6-3, 6-1.
Li Na (8), China, def. Akgul Amanmuradova, Uzbekistan, 6-4, 6-4.
Petra Kvitova (3), Czech Republic, def. Francesca Schiavone, Italy, 6-2, 6-2.
Sam Stosur (5), Australia, def. Julia Goerges, Germany, 6-2, 2-6, 6-3.
Victoria Azarenka (1), Belarus, def. Andrea Petkovic, Germany, 6-2, 4-4, retired.
Angelique Kerber, Germany, def. Caroline Wozniacki (6), Denmark, 6-1, 6-2.