Angelique Kerber moved within two victories of the world number one ranking when she beat Spain's Carla Suarez Navarro in their quarter-final at the Western & Southern Open in Cincinnati on Friday.
The German second seed overcame a slow start before finding some form and outlasting ninth seed Navarro 4-6 6-3 6-0 in a match that never reached any great heights in front of a small crowd in steamy afternoon heat.
Kerber's task was perhaps made easier by an apparently injured Navarro, who moments after losing withdrew from her doubles match, citing a thigh complaint.
If Kerber wins on Sunday, she will displace Serena Williams as world number one, ending the American's run of 183 consecutive weeks at the top.
Williams was a late withdrawal from Cincinnati, citing a shoulder injury.
To overtake Williams, Kerber must first get past Romanian Simona Halep or Poland's Agnieszka Radwanska in the semi-final on Saturday. Halep and Radwanska play their last eight match later on Friday.
Kerber has played a heavy schedule over the summer, and it showed in her sluggish start on Friday.
"It was really difficult to go out there," she told the WTA website. "I'm playing like I don't know which day ... every day, it's the same. When I'm waking up it's the same routine I have ... Of course it's really hot out there."
The other semi-final will pit fourth seed Garbine Muguruza against 15th seed Karolina Pliskova.
Spaniard Muguruza, the French Open champion, beat Hungarian qualifier Timea Babos 6-4 6-3, while Czech Pliskova upset Russian seventh seed Svetlana Kuznetsova 6-3 4-6 6-2.
Pliskova credited her victory to improved serving as the match progressed.
"In the first set I tried to play aggressive and not give her time to dictate the game," she said of Kuznetsova.
"My serve wasn't that good in the first two sets, but in the third it improved a lot."