Ashleigh Barty is a win away from regaining the world's top ranking after producing another spirited comeback to reach the semi-finals of the Cincinnati Open.
Barty fought back to beat Maria Sakkari 5-7 6-2 6-0 to advance to the final four, as Japanese star Naomi Osaka lost her grip on the top ranking with a quarter-final retirement against Sofia Kenin.
If Barty beats Russian veteran Svetlana Kuznetsova next, the Australian will move into the top spot for the US Open.
Osaka had much bigger concerns after dropping out of her semi-final match because of a knee injury.
After splitting sets with Sofia Kenin, Osaka took a time-out early in the third to have her left knee examined and wrapped. She played only two more points before retiring while trailing 6-4 1-6 2-0, sending the American into the semi-finals for the second straight week.
Last week in Toronto, Kenin knocked off Barty on her way to the last four, allowing Osaka to jump ahead of Barty for No.1. Now they're in position to swap spots again.
Barty is feeling good about her play with the US Open 10 days away.
"Still not quite there, but we're getting better and better each match," the French Open champion said.
"That's the beauty of being able to stay alive in the tournament is to give myself an opportunity to keep getting better.
"I'm still aiming to obviously do very well here, but the big picture is New York in 10 days' time."
She needs some work on first sets, though.
Barty got ahead 5-2 in her opening set Friday (Saturday AEST) and then got sloppy, letting it slip away as Sakkari became more aggressive.
It was the second straight day that Barty rallied after dropping a first set. She fought off a match point during a three-set win over Anett Kontaveit, showing poise that she'd need to draw on again against Sakkari.
"There is certainly no panic, where it gets to the stage where I'm kind of overwrought or can't think straight," Barty said.
"I'm still able to, more times than not, have the ability to relax and kind of stay clear and come back to what works."
Kuznetsova reached her first semi-final of the season by beating third-seeded Karolina Pliskova 3-6 7-6 (2) 6-3.
Kuznetsova is playing only her ninth event after missing the start of the season because of a knee injury.
The other semi-final will be fought out between Kenin and Madison Keys, who overcame veteran fellow American Venus Williams 6-2 6-3 on Friday (Saturday AEST).