The fifth-seeded Zverev overcame a two-set deficit to beat No. 20 seed Pablo Carreno Busta 3-6, 2-6, 6-3, 6-4, 6-3 and reach the men's title match at Flushing Meadows. He became the first man to win a U.S. Open semifinal match after a 2-0 set deficit since Novak Djokovic in 2011, when the Serbian did it against Roger Federer.
Zverev, 23, also is the youngest male finalist at any major tournament since Djokovic, who reached the U.S. Open final in 2010 at age 23.
"Mentally, I stayed in it. ... A lot of players would have gone away," Zverev said. "There's no easy matches anymore. Sometimes you have to dig deep. Today, I dug deep, dug very deep.
"... I started taking the ball much earlier. I started taking the ball more on the rise. I started giving myself the chance to be the one that is aggressive. I think in the first few sets, Pablo was the one dictating much more than I did. This is how the match slowly turned around."
Later Friday, the second-seeded Thiem clinched his spot in the men's final with a straight-sets victory over No. 3 seed Daniil Medvedev. Thiem grinded out a 6-2, 7-6 (7), 7-6 (5) win over the 24-year-old Russian to advance to his first U.S. Open final.
Both Thiem and Zverev are searching for their first Grand Slam trophies.