Ash Barty had to battle back from a set down to keep her quest to reclaim the World No.1 ranking alive at the Cincinnati Open.
The Australian top seed secured a semi-final showdown with Svetlana Kuznetsova after a come-from-behind win over Maria Sakkari.
Barty, who also had to recover from losing the opening set before edging Estonian Anett Kontaveit in the previous round, surged to a 4-1 advantage in the first set on Friday 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."
The French Open champion, who fought off a match point against Kontaveit on Thursday, said she had again backed herself to fight back when the pressure was on.
"There is certainly no panic, where it gets to the stage where I'm kind of overwrought or can't think straight," she added.
"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."
Barty, currently world No.2, can reclaim the world top spot by reaching the tournament final in the US Open warm-up event.
But she said: "I'm just keeping my eye on the ball and if I win matches, the ranking will take care of itself."
Her last-four opponent Kuznetsova reached her first semi-final of the season by beating third-seeded Karolina Pliskova 3-6 7-6 (7-2) 6-3 later in the day.
Kuznetsova is playing only her ninth event after missing the start of the season because of a knee injury.
Playing on a hot, humid afternoon, she got treatment on her upper right leg after the second set and twice again during the final one.