Fils and Shelton set for friendly fire in Basel semi-finals
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 semi-final 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 Basel."
Another rising French player, Giovanni Mpetshi Perricard, battled his way through to the semi-finals on the other side of the draw, edging past the Canadian Denis Shapovalov 6-7 (7/9), 6-3, 7-6 (7/3).
The 21-year-old, who won in Lyon in the spring, continued his impressive serving.
Having fired down 21 aces in the first round and 22 in the second, he slammed down 27 on Friday, albeit in three sets, the fastest of which fizzed down at 235 km/h.
He will face fellow 21-year-old Holger Rune, the number four seed, in tthe semis after the Dane eased past David Goffin 6-2, 6-4 in 79 minutes.
