Ana Ivanovic was forced to retire at 2-1 down in the first set of her semifinal match against Kim Clijsters with a foot injury.
"Just on that one forehand when I went to step around it, I just felt so much pain in my foot and I couldn't step on it anymore," she said. "I tried bit of tape, but I could not put any weight on it.
"I'm obviously disappointed the way it finished because I felt like I had really good run."
Cincinnati was Ivanovic's third semifinal of the year, with the other two coming in Brisbane and Rome.