Top seed Ash Barty rallied from a set down to make the Cincinnati Masters semi-finals but world number one Naomi Osaka withdrew from her quarter-final with an apparent knee injury on Friday.
Osaka was trailing American Sofia Kenin in the third set when the Japanese player sought treatment on her left knee.
After briefly returning to play, Osaka retired. Kenin was leading 6-4 1-6 2-0.
The withdrawal comes less than two weeks before the start of the U.S. Open, where Osaka is the defending champion.
“This isn't the way I wanted to win,” Kenin said in a courtside interview.
“I wish Naomi a speedy recovery so she can get ready for the U.S. Open. She's a great player and I have a lot of respect for her.”
Australian Barty, the number two ranked player, rallied for a 5-7 6-2 6-0 victory over Greek Maria Sakkari.
Barty will face Russia's Svetlana Kuznetsova in Saturday's semi-finals and a victory would return her to top of the world rankings.
Kuznetsova outlasted Czech third seed Karolina Pliskova 3-6 7-6(2) 6-3, striking 30 winners and breaking Pliskova three times.
The 34-year-old Kuznetsova was down a set with her Czech opponent serving for the match but found a way to turn things around.
Barty also needed a comeback.
The Australian surged to a 4-1 advantage over Sakkari in their first set and appeared on her way to an easy triumph.
But Sakkari had other ideas.
The Greek, using a variety of backhand winners that resulted in mounting unforced errors by Barty, wrestled control to take the set.
In explaining the turnaround, Barty said, “I started the match well and I went away from what was working. Maria took some risks that paid off.”
The second saw a reversal of form with Barty cleaning up her errors and effectively employing a backhand slice that deserted her in the opener.
“It was important for me at the start of the second set to go back to basics and what worked before,” the 23-year-old Barty said. “I had to serve well, and not give her too many looks at second serves. I tried to bring in my slice to change the tempo of the match.”
With 11 winners and just four unforced errors she easily took the set.
Sakkari was no match in the third as Barty raced through for the win.
