World number two Elena Rybakina had to quit her quarter-final at the ATP-WTA Cincinnati Masters, retiring while trailing Iga Swiatek 4-1 in the opening set on Thursday.
The Kazakh suddenly stopped play, unable to put any weight on her left heel, and after a brief intervention from her trainer was forced to retire after one more point.
Rybakina's injury -- coming 10 days before the US Open begins -- sends Swiatek into a Cincinnati semi-final with third-seeded Jessica Pegula, who defeated Amanda Anisimova 6-4, 2-6, 7-6 (7/4).
Had two-time Grand Slam winner Rybakina reached the Cincinnati final she would have risen to number one in the world for the first time in her career, following incumbent Aryna Sabalenka's round-of-16 exit.
Belarus's Sabalenka was stunned by Czech Sara Bejlek, ranked 35th, in a gritty match that finished in the early hours of Thursday morning -- the latest in a string of setbacks for the four-time major champion.
"Honestly, I was feeling anxious just looking at this situation," Swiatek said in an on-court interview. "I hope that she will be fine. Obviously the net tournament is the priority.
"I know this one is important as well, but US Open is US Open and I hope she's going to be ready for that. She's also super-close to being number one in the world, so it's a shame."
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