The Francis-Henri Graffard-trained Erdenali bounced back to form in the Qatar Prix du Moulin de Longchamp as Gstaad could only finish third.
The Aga Khan Stud-owned winner was only third in a minor Group Three at Chantilly when last seen in July, though was unlucky on that occasion, and made up for the reversal in style with a win at the first attempt in Group One company.
Having been two lengths ahead of the re-opposing Opera Ballo in the Sussex Stakes last time, the Aidan O'Brien-trained colt Gstaad was sent off a strong favourite, but the mount of Ryan Moore failed to pick up as well in the home straight as Erdenali and Ten Bob Tony who were finishing best.
However, it soon became clear that the mount of Mickael Barzalona had more in reserve than his Ed Walker-trained rival, and he came nicely clear late on.
Opera Ballo was taken on by Dorset for the early lead and was too keen as a result under William Buick, eventually finishing well held.
Nemone Routh, racing manager for the owners, said: "He's not a horse who shows very much in the morning. You wouldn't be impressed but it all comes together in the afternoons.
"He should have been unbeaten. We wanted to find out if he is good enough. He's really grown up. He's proved that he's special and that he's a real Group One horse.
"His granddam won this race a few years ago and we are so lucky that we have her son at stud."
Ed Walker, trainer of the runner-up, added: "Thrilled and very proud of him again
"He's had an amazing season and we've been a bit more aggressive this year in terms of how much he's racing and on different ground.
"This year he's really stepped it up a gear. A bit gutted – I thought we had it – and we've probably been beaten by a very good horse.
"The plan was always the QEII, and then maybe the Breeders' Cup."