Taylor Fritz outlasts Alexander Zverev to book spot in ATP Finals showpiece
The American fought off numerous break points in a cliffhanger deciding set to win 6-3, 3-6, 7-6(3), ending a two hour 20 minute duel with a stunning forehand winner.
Fritz will face either home favourite and world number one Jannik Sinner or Norway's Casper Ruud in Sunday's final.
Zverev had cruised through the group stage in majestic fashion and looked in the form to win the prestigious title for a third time, but his eight-match winning streak was ended by the resolute American who is ending the year on a high.
World number five Fritz had won his previous three matches against Zverev and was the better player in the opening set as the German dropped his serve for the first time this week.
Zverev hit back though to set up a deciding set and had enough chances to have taken the win. But Fritz dug deep to hold serve from 0-40 at 2-2 and then saved two break points at 5-5.
Fritz got ahead in the tiebreak and admirably held his nerve to match compatriot James Blake who reached the final in 2006. No American has won the title since Pete Sampras in 1999.
"I felt like I played almost a perfect first set but it was tough in the second and third," said Fritz, who will end the year at a career-high world number four if Sinner beats Ruud.
"I found myself in some tough spots in the third, both of us really started to get on each other's serve even more. I told myself just to give him nothing from the back."
Zverev did not manage to find the level he achieved during the group phase but still looked the favourite in a third set full of attritional rallies from the baseline.
Fritz looked in big trouble at 2-2 when some errors looked like proving costly but he managed to hold serve.
Zverev then found himself in a big hole at 3-4 down as he went 15-40 but fought off the break points and then another one after a double fault to stay level.
Fritz, who surprisingly matched Zverev from the baseline throughout an enthralling match, showed great resilience towards the end and once he reached the tiebreak the odds swung in his favour as his opponent began to run out of steam.
"I've been playing the top guys at the big events a lot lately," Fritz, who lost to Sinner in the US Open final, said. "I'm getting more comfortable and confident in those moments."