Garbine Muguruza and former world number one Angelique Kerber forced to retire at Porsche Grand Prix

But third seed Elina Svitolina was able to take advantage of the third retirement of the day

Garbiñe Muguruza retired at the end of the first set (Getty)

Second seed Garbine Muguruza and former world number one Angelique Kerber were forced to retire midway through their respective matches at the Porsche Grand Prix in Stuttgart.

Muguruza had been out-lasted 7-5 in a tight first set against Anastasia Pavlyuchenkova before she abruptly withdrew with an unspecified complaint.

And Kerber lost the first eight games of her match against Estonian 22-year-old Anett Kontaveit before pulling out, revealing she had been battling the effects of a right thigh injury.

Kerber retired with an injury (Getty)

Kerber told wtatour.com: “I was trying everything to go out there. I was thinking of playing for the because it was my home tournament, but in the end it was too much."

Fourth seed Jelena Ostapenko out-hit Zarina Diyas of Kazakhstan 6-3 6-0 in just 63 minutes, and fifth seed Karolina Pliskova beat qualifier Veronika Kudermetova 7-6 (7/4) 6-3.

Third seed Elina Svitolina took advantage of a third retirement of the day, when Czech qualifier Marketa Vondrousova retired trailing 2-6 6-1 3-2, while Caroline Garcia took three tight sets to see off Ukraine's Marta Kostyuk 6-1 3-6 7-5.

Svitolina is through to the next round (Getty)

Top seed Caroline Wozniacki survived a scare at the Istanbul Cup, fighting back from a set down to beat world number 93 Sara Errani 5-7 6-3 6-3 and book her place in the last eight.

Donna Vekic was a comfortable 6-1 6-2 winner over Ajla Tomljanovic while there were also second-round successes for Maria Sakkari, Polona Hercog and qualifier Arantxa Rus.