Arthur Fils made it through to the semifinals of the indoor event in Basle with an impressive 7-6 (7/5), 6-3 win over third seed Stefanos Tsitsipas on Friday.
The 20-year-old Frenchman, who will next meet close friend Ben Shelton, his doubles partner for the week, has now won 13 consecutive matches at this level, including title runs in Hamburg and Tokyo this year.
"I was very focused" said Fils. "I missed a crazy (first) set point, but I am very happy with how I dealt with the tie-break and I think I played pretty good in the second set as well."
Defeat was a major setback to Tsitsipas' hopes of qualifying for a sixth consecutive appearance at the ATP Finals in Turin.
The Greek won won the tournament on his debut in 2019 but is currently 12th in the rankings in the Race To Turin.
Only the top eight qualify and at the moment Tsitsipas is 745 points adrift of Andrey Rublev who is in that eighth spot.
The Russian, however, also had a bad day, beaten by Shelton 7-5, 6-7 (3/7), 6-4 with the American setting up a semifinal showdown for the doubles partners.
"He (Fils) is one of my best friends on the Tour, and I see him as one of my main rivals out of the young guys," said Shelton.
"He's playing incredible tennis at the moment, and there is no one I would rather match up against on a Saturday here in Basle.