France, Sept. 1 -- Fils, who had taken the first set 6-4, served for the second set against the 28-year-old Greek at 5-3 up.

But he fluffed the opportunity. Tsitsipas then held serve to level at 5-5 before winning the set on a tiebreak.

Tsitsipas, a former world number three, surged away to take the match in four sets.

"I don't really know what happened," said Fils who entered the US Open as 10th seed and one of the favourites for the title following his run to the quarter-finals at the Montreal Masters 1000 and his title at the Cincinnati Masters 1000.

"I don't have the answer," added the 22-year-old. "But the energy went down at the end of the second set and I never felt again a good energy.

"At my level, to make such a mistake is ...