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.
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To overtake Williams, Kerber must first get past Romanian Simona Halep who defeated Poland's Agnieszka Radwanska 7-5 6-1 to reach the semi-final.
Kerber and Halep last met in the quarter-finals of Wimbledon, where the German prevailed in two tight sets, but Halep has not lost a match since en route to victories at the Bucharest Open and Rogers Cup in Montreal.
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."
(Reporting by Andrew Both in Cary, North Carolina; Editing by Greg Stutchbury/Sudipto Ganguly)
