With No.2-ranked Rybakina serving at 1-4, 40-40 in Thursday's clash, she took a medical timeout to have trainers address her left ankle area and retired from the match moments later when unable to put full weight on her leg while attempting to serve..
"Of course I feel not the greatest," Rybakina said. "It's painful to walk. We'll know more tomorrow when we have the MRI. It's been bothering me for a long period of time but felt like I had it under control.
"It was feeling OK before (Thursday's) match. Unfortunately one wrong movement and something happened."
With world No.1 Aryna Sabalenka suffering a shock loss to Sara Bejlik in the previous round, the 27-year-old Rybakina could have risen to top spot in the rankings if she had reached the final.
Swiatek, the defending champion, has reached at least the semi-finals in each of her last four appearances in Cincinnati and beat Rybakina in the semis last year. She'll face American Jessica Pegula in the semi-finals.
"Not the way you want the match to end," Swiatek said. "I felt really sorry for her but hopefully it's nothing serious and she's going to be ready for the US Open."
Pegula beat fellow American Amanda Anisimova 6-4 2-6 7-6 (7-4) for her tour-leading 44th win of the season. It's her second trip to the semi-finals in Cincinnati, where she lost to Sabalenka in the final two years ago.
Four American men reached the quarter-finals in Cincinnati for the first time since 1995, but Flavio Cobolli knocked out Tommy Paul 2-6 6-3 6-4, to reach his fifth semi-final this year and rise to a career-best No 6 in the rankings. Cobolli is the second Italian man to reach the semis in Cincinnati, joining Jannik Sinner.
Cobolli will next face 22-year-old Arthur Fils who beat Thiago Agustín Tirante 6-3 6-2 to become the sixth man born since 2000 to reach three ATP Masters 1000 semi-finals.
