The British number two is now sitting at a career-high of 73rd in the ATP World Rankings, and the left-hander has continued to impress with an extended run on his first outing since Wimbledon.
World number 73 Norrie led 7-5 3-0 when the Australian pulled out with what looked like a recurring hip problem, drawing boos from the crowd. The 33-year-old, ranked ninth in the world, lost only three points on serve in the set while breaking Zverev twice.
The top seed dropped the first set against eighth seed Harrison, but fought back to clinch a 5-7 6-3 6-4 victory.
Injury-prone Nick Kyrgios has been booed off court after retiring hurt during his quarter-final against Brit Cameron Norrie. I played well today.
It will be an Isner-Harrison final in the ATP World Tour 250-level tournament for the second consecutive year, after Isner triumphed last year.
In the other quarterfinals Friday, Isner will face seventh-seeded Mischa Zverev of Germany, and eighth-seeded Ryan Harrison, the losing finalist past year, will meet Chung Hyeon.
"I always feel comfortable playing here. the court suits my game style", he said yesterday after defeating American Noah Rubin.
Isner has 14 ATP Tour titles, also winning the Miami Open in April. "There are no secrets there, I know a lot about him".
Isner appeared to be wilting in the heat but Harrison could not hammer home his advantage, as his opponent squirmed out of trouble by saving four break points.
They also met past year in the Hall of Fame Open final, with Isner winning in straight sets. Now I'm sort of playing in my backyard; he beat me in his backyard.
"It was a little nervy from both of us there at the end".