Kenin has now won a tour-leading 15 matches at majors this year. She started the year by winning the Australian Open and then reached the fourth round at the US Open last month.
Collins hadn’t dropped a set in her three previous meetings with Kenin.
Collins won a rain-delayed three-set quarterfinal over Ons Jabeur a day earlier. She left the court for a medical timeout while trailing Kenin 4-0 in the third set.
It wasn’t immediately clear what was bothering Collins but she told the trainer that it was “unbearable.”
Kenin’s semifinal opponent will be two-time Wimbledon champion Petra Kvitova, who overcame some potentially costly double-faults to beat 66th-ranked Laura Siegemund 6-3, 6-3 and reach the semifinals for the second time earlier Wednesday.
Kvitova normally dictates play with her big serve and forehand, but she double-faulted multiple times when she was broken twice in the second set.
The seventh-seeded Czech player bounced back each time and is back in the last four at Roland Garros for the first time since 2012. She lost to eventual champion Maria Sharapova that year.
Kvitova has not dropped a set in her five matches at this year’s tournament.
Instead it was Siegemund who had the most costly double-fault. It came on Kvitova’s second match point.
Siegemund also missed a series of drop-shot attempts into the net. The German took a medical timeout to have her lower back treated after dropping serve to give Kvitova a 3-2 lead in the second set.
The Associated Press contributed to this report.