Injury ends Kerber's hopes
Local favourite and former champion Angelique Kerber is out of the Porsche Tennis Grand Prix in Stuttgart after suffering an injury
The former world No.1 was trailing Anett Kontaveit 6-0 2-0 when she was forced to retire with a right thigh issue. "I was feeling it a lot in the warm-up," Kerber told the WTA website.
"It started a few days ago, but this morning, I was feeling it more and more. I was trying everything to go out there. I'm here in Stuttgart, my home tournament, so I was trying everything, thinking I could play for the fans.
"At the end, it was too much and I started to feel the pain a lot in the first two games, where maybe I was sliding a little bit differently."
Garbine Muguruza also failed to complete her match on an injury-hit day in Germany. The Spaniard, citing a back problem, quit after conceding the first set 7-5 to Anastasia Pavlyuchenkova.
No.3 seed, Elina Svitolina, did manage to advance, however she was also helped by a retirement. Marketa Vondrousova felt unable to continue after five games of a deciding set.
In matches that were completed, Coco Vandeweghe took out defending champion, Laura Siegemund, 6-4 4-6 6-3 and Caroline Garcia squeezed beyond teenage sensation, Marta Kostyuk, 6-1 3-6 7-5.
Jelena Ostapenko, who dazzled on clay last year, thumped Zarina Diyas 6-3 6-0 while Karolina Pliskova dismissed Veronika Kudermetova 7-6(4) 6-3.
The Latvian and the Czech are scheduled to meet in the last eight on Friday.
At the Istanbul Open, Caroline Wozniacki squeezed beyond Sara Errani 5-7 6-3 6-3, Donna Vekic thrashed Ajla Tomljanovic 6-1 6-2 and Maria Sakkari skipped past Aleksandra Krunic 6-3 6-1.
Arantxa Rus and Polana Hercog are also through to the quarter-finals.