STUTTGART, Germany (AFP) - Back-to-back retirements within an hour of each other on the showcase court left the Stuttgart Grand Prix struggling on Thursday (April 26) as Angelique Kerber and second seed Garbine Muguruza quit injured.

The mid-match pullouts made for an abbreviated evening session and likely left top seed Simona Halep with a clear path to the weekend.

Kerber retired with a right thigh problem trailing 6-0, 2-0 to Anett Kontaveit of Estonia while Muguruza stopped with lower back pain after losing the opening set 7-5 to Anastasia Pavlyuchenkova of Russia.

Three of the day's seven matches ended in retirements - earlier third seeded Elina Svitolina of Ukraine was 2-6, 6-1, 3-2 ahead of Marketa Vondrousova when the Czech qualifier quit with right thigh trouble.