Angelique Kerber and Garbine Muguruza both withdrew early from their second-round matches at the Stuttgart Open on Thursday, disappointing the fans in attendance at the evening session.

World number 12 Kerber, a champion at this WTA Premier event in 2015 and 2016, was struggling with a thigh injury against the unseeded Anett Kontaveit and decided enough was enough at 6-0 2-0 down.

Kerber had taken a medical time-out at 5-0 in the first, but treatment failed to sufficiently ease the problem.

The former US Open champion was forced to bow out to present Kontaveit with a safe passage to the quarter-finals, where she will take on Anastasia Pavlyuchenkova, who was next on court against second seed Muguruza.

The pair traded breaks in games eight and nine of the opener before, with Muguruza serving at 5-6 down, Pavlyuchenkova ripped a forehand up the line for break point and then watched the Spaniard send one long.

Although Muguruza showed no obvious signs of discomfort, the world number three got up from her chair before the second set got under way and shook hands with the Russian while the Stuttgart crowd voiced their disapproval at a second straight retirement.

Defending champion Laura Siegemund - who lost to Kerber in the final two years ago - saw her hopes of successive crowns ended by a 6-4 4-6 6-3 defeat to Coco Vandeweghe, who will face top seed Simona Halep.

Kerber and Muguruza were not the only retirees on the day as Elina Svitolina was 2-6 6-1 3-2 to the good over Marketa Vondrousova before the Czech pulled out.

Svitolina's quarter-final opponent comes in the form of Caroline Garcia, while Karolina Pliskova will face Jelena Ostapenko after their respective wins on Thursday.