Aryna Sabalenka knocked out of Stuttgart Open by Marketa Vondrousova

Sabalenka had won her last four matches with Vondrousova, and after winning the opening set, the Australian Open champion looked on course to progress.

Wimbledon champion Vondrousova led 4-1 in the second set before Sabalenka pulled it back to 4-3 but the Czech sixth seed broke for a third time and held serve to force the decider.

Sabalenka went 5-4 ahead in the final set before Vondrousova reeled off three games in a row to reach the semi-finals where she will meet either American third seed Coco Gauff (20) or Marta Kostyuk (21) of Ukraine.

"It was a very tough match, and I think a great one, too," Vondrousova said.

"She's a great player so I just tried to stay in the rallies. She hits very fast so it's tough to do something. But I stayed calm in the crucial moments."

Sabalenka reached the quarter-finals after her good friend Paula Badosa (26) was forced to retire through injury on Wednesday, with the match tied at one set each and 3-3 in the decider.

Fourth seed Elena Rybakina (24) also reached the semi-finals after overcoming the challenge of in-form Jasmine Paolini (28) 6-3, 5-7, 6-3.

Paolini won the Dubai Tennis Championships in February and rose to a career-high 14th in the world rankings and reached the last 16 at Indian Wells in March.

After winning the opening set, Rybakina had let slip a 4-2 lead in the second and came back from an early break by the Italian in the final set.

"I'm happy with my energy," Rybakina said.

"Of course, I had chances in the second set where I could have closed it out but overall, I think it's just a matter of matches and feeling better and better on the court."

Rybakina will face either world number one Iga Swiatek (22), who saw off Britain's Emma Raducanu (21) 7-6, 6-3.