Favourite Zarkava produced a storming finish to take the Prix de l'Arc de Triomphe at Longchamp.
The three-year-old filly, ridden by Christophe Soumillon, was behind early on but showed superb acceleration in the closing stages to win comfortably.
"She's beautiful, it was perfect," trainer Alain de Royer-Dupre told BBC Sport. "She's the best horse I've had."
Mick Channon's Youmzain ran another game race to finish runner-up for the second year in a row.
"I tried to go up the fence but it didn't quite happen as the gap closed," said Youmzain's jockey Richard Hills. "He's run a great race anyway and he's probably even better than last year."
"I was not worried, you don't get horses like this every day"
The stewards ruled that It's Gino and Soldier of Fortune shared third place in a photo finish.
Zarkava, who went into the race on the back of six straight wins, was a major doubt because of earlier concerns over the soft going at Longchamp.
But only one millimetre of rain fell overnight at the Paris track and her participation was confirmed just before the start.
Drawn in stall one on the inside rail, there were worries about her leaving the stalls after she was left at the start in her prep race.
She struggled at the off again in the main race, lurching to her right with her Belgian jockey working hard to stay in the saddle.
But Zarkava worked her way back through the field and as the riders turned into the straight, she was lying in fourth.
It looked as if it might be too much to ask for her to fight her way through but the gaps appeared and Zarkava charged home to become the first filly to win the race since Urban Sea in 1993.
"When I saw the filly in the paddock and in the parade I was sure she was OK," added Royer-Dupre. "I know she has great acceleration.
"She handled the situation very well. I was not worried, you don't get horses like this every day.
"She had a very easy race. It's great to train a filly like that.
"I was confident during all of the race except the start where she didn't go straight, but after that it was OK. Every day you are anxious in case she has a problem or if something goes wrong.
606: DEBATE
Irish trainer Aidan O'Brien, who is chasing a world record of 26 Group One wins in a season, remained on 21 victories with Soldier of Fortune his best-place finisher of three entries.
One of his other runners, Red Rock Canyon, made the early running but trump card Duke of Marmalade - one of the pre-race fancies - never got going.
Sunday's meeting started in chaotic scenes with a false start called in the Prix de l'Abbaye after Fleeting Spirit's stalls gate refused to open.
Most of the runners raced more than three furlongs while Hungary's Overdose blazed a fruitless trail down the five-furlong course.
Officials eventually decided that the race would be re-run at 1730 BST.
"We're not going to run again," said Overdose's trainer Sandor Ribarszki. "I'm devastated. I've never felt as low as this in my life.
"I didn't travel 1,700 miles to be ridiculed like this."
Frankie Dettori, who was aboard Dandy Man, added: "I realised about halfway that it was a false start, but to try and pull up these sprinters at full speed is almost impossible."
Dandy Man was left out of the re-run race, which was won by Marchand D'Or and jockey Davy Bonilla.
Prix de L'Arc de Triomphe result:
1. Zarkava (C Soumillon) 13-8f
2. Youmzain (R Hills) 12-1
3= It's Gino (T Thulliez) 150-1
3= Soldier of Fortune (S Heffernan) 9-2
16 ran
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