World number 22 Jack Sock beat qualifier Filip Krajinovic to win his first Masters 1000 title in Paris and qualify for the ATP Tour Finals.
The 25-year-old American came from a set down to beat the Serb, 77th in the world rankings, 5-7 6-4 6-1.
Sock, seeded 16th, is the first American to win the tournament since Andre Agassi in 1999.
Victory meant he took the final place at the season-ending ATP Tour Finals from Spain’s Pablo Carreno Busta.
Krajinovic beat ninth seed John Isner and Wimbledon semi-finalist Sam Querrey on his way to his first ATP Masters final.
He was broken by Sock in the sixth game of the opening set but responded immediately and took the first set when his opponent netted a forehand.
The American found his rhythm, serving nine aces and winning 79% of points on his first serve, as he won in one hour and 59 minutes.
Sock will move into the world’s top 10 for the first time when the new rankings are released on Monday, two days before the draw for the ATP Tour Finals, which take place in London from 12-19 November.
Former world number one Novak Djokovic, who ended his season in July with an elbow injury, will fall out of the world’s top 10 for the first time since 2007.