Taylor Fritz became the first American to reach the ATP Finals title match in 18 years as he defeated Alexander Zverev in a thrilling three sets.
Fritz progressed to Sunday's final in Turin, Italy, with a 6-3 3-6 7-6 (7-3) victory - his fourth over the German second seed this year alone.
He will face the winner of tonight's second semi-final between world number one Jannik Sinner and Norway's Casper Ruud.
Not since 2006, when James Blake lost to Roger Federer, has a US player reached the final of the season-ending tournament.
"It was tough in the second and third sets," said fifth seed Fritz.