The 18-year-old Michelsen, playing in his second career ATP tournament after winning a Challenger Tour event in Chicago last week, advanced to Sunday's grass-court final against No. 2 seed Adrian Mannarino, who beat fellow Frenchman and third-seeded Ugo Humbert, 6-4, 6-3.
In an interesting twist, Michelsen, who signed and committed to Georgia, beat Isner, who attended the school and reached the 2007 NCAA singles final with the Bulldogs.
"When he was about to warm up serves, I was like, 'Wow, I'm playing this guy!''' said Michelsen, from Aliso Viejo, California. "I grew up watching him. Pretty cool."
The 38-year-old Isner, who won Newport in 2011, 2012, 2017 and 2019, has relied on his overpowering serve in his successful run at the event. He entered with a 28-6 record at the Hall.
When the match ended, Michelsen tossed his racket and raised both hands to the roar of the crowd. After shaking hands with Isner at the net, the teenager turned and raised both fists to the crowd's cheers.
"It doesn't feel like I'm playing in the final. Maybe that's why I'm playing so loose," said a smiling Michelsen. "I was honestly guessing right when I needed to, to get some of the serves to get that break early in the second."
Isner had lost seven straight matches before making his run this week.
Mannarino is looking for his second ATP tour title to pair with his victory at Hertogenbosch in 2019.
"I took my chances on the big moments and it worked," Mannarino said. "It's on Newport grass. Anything can happen."
The tournament is held in conjunction with enshrinement ceremonies into the Hall of Fame.
Esther Vergeer, 42, a dominant wheelchair player who captured 21 Grand Slam singles titles and a seven-time Paralympic gold medalist, led the Class of 2023. She was inducted with American Rick Draney, 61, who won 12 singles titles and six in doubles before the Grand Slam era of wheelchair tennis.
Vergeer held the No. 1 spot in the world rankings for 668 weeks from 2000 to 2013 while winning 470 straight singles matches. She won 96% of her singles matches in all, and also claimed 136 doubles titles.
Draney has been credited with bringing quad tennis -- a classification that accounts for impairment in the arms, as well -- to the Paralympics and other top tournaments.