"He is a tough player. I played well today. I needed to," Isner said. "It was a weird match. The balls were bouncing everywhere, it was tough to control. I was happy to get through in three sets."
Seeking his fifth victory at Atlantic Station in the last six years, the top-seeded Isner set up a match against fourth-seeded Matthew Ebden of Australia. Ebden beat Marcos Baghdatis of Cyprus 6-3, 6-2.
The 33-year-old Isner lost only two first-serve points against the seventh-seeded Zverev, won all but three of his service points in the third set and broke twice.
Isner improved to 29-4 in the event, winning his second match after a 6-hour, 36-minute loss — with a 26-25 fifth set — to Kevin Anderson in the Wimbledon semifinals. The 6-foot-10 former University of Georgia star won the Miami Open in April for his 13th ATP Tour title.
Isner is 7-1 in the semifinals, with the lone loss coming to Andy Roddick in 2012. Isner's only other losses in the event came in the finals, to Mardi Fish in 2010 and 2011 and Nick Kyrgios in 2016. Isner won titles in 2013, 2014, 2015 and 2017.
The 30-year-old Ebden is chasing his first ATP World Tour title. He's 1-3 against Isner, winning in January in the Australian Open.
"I am definitely playing better than I was in the beginning of the year. I was struggling a little bit," Isner said. "To his credit, he played much better than me that day in Australia. He had the home crowd on his side. It helps a lot. Now, I'm playing in my backyard and he beat me in his backyard. We'll see if I can turn the tide."
They also met last year in the Hall of Fame Open final, with Isner winning in straight sets.
"This is exactly what the doctor ordered for me," said Harrison, the runner-up last year. "It was a little nervy from both of us there at the end."
Harrison will face Cameron Norrie of Britain. Norrie led the second-seeded Kyrgios 7-5, 6-3 when the Australian retired.