Taylor Fritz became the first American since 2006 to reach the title match at the ATP Finals after winning a slugfest against world number two Alexander Zverev in on Saturday.
The 27-year-old 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.
