In-form fourth seed Victoria Azarenka was forced to retire from her second-round match at the Stuttgart Grand Prix with a shoulder injury.
The Belarusian, who had won her last two events, pulled out after winning the first set against Julia Goerges.
French Open champion Francesca Schiavone made a poor start to her clay season with a 6-1 6-3 defeat by Poland's Agnieszka Radwanska.
But world number one Caroline Wozniacki beat Slovakia's Zuzana Kucova 6-1 6-2.
The Dane next face Germany's Andrea Petkovic, a 3-6 6-1 6-3 winner over Jelena Jankovic on a day that saw four home players qualify for the quarter-finals.
Along with Petkovic and Goerges, Sabine Lisicki downed Australian Open runner-up Li Na 6-4 7-5 and Kristina Barrois knocked out France's Marion Bartoli 6-4 6-2
Second seed Vera Zvonareva rallied to beat Anastasia Pavlyuchenkova 3-6 6-4 6-2, and French Open finalist Samantha Stosur suggested she is starting to rediscover some form as she defeated Daniela Hantuchova 6-4 2-6 6-4.
Azarenka had looked well placed to challenge for the title this week on the back of victories in Miami and Marbella, but could not shake off a shoulder problem.
"I picked up a shoulder injury during Fed Cup," she said. "I have been trying to rehab the last few days and tried my best here but it keeps getting worse.
"I didn't want to push it any further at the start of the clay season. I'll go back to do some therapy in Monaco so hopefully I'll have time to recover and play Madrid."