Wild Illusion claimed further top-level success in the Prix de l'Opera at ParisLongchamp.
The daughter of Dubawi won the Prix Marcel Boussac on Arc day last season.
She began the current campaign by finishing fourth in the 1000 Guineas at Newmarket, second in the Oaks at Epsom and second again in the Ribblesdale Stakes at Royal Ascot.
Wild Illusion rediscovered the winning habit when dropped to a mile and a quarter for the Nassau Stakes at Goodwood and she was a 15-8 favourite on her return to France.
Ridden by William Buick, Charlie Appleby's filly was always well positioned and battled her way to the lead halfway up the home straight.
The Aidan O'Brien-trained Magic Wand kept her honest in the final furlong, but Wild Illusion was always doing enough to hold her at bay.
Buick said: "She's a filly that's always liked a bit of cut in the ground, so I was pleased to see the rain come.
"She's been a great filly to ride at the top level throughout the season and I was delighted with the way she buckled down at the finish."
Appleby said: "She's a tough filly and a credit to herself and the team.
"She's been a star and a filly we've always been fond of, ever since the start of her two-year-old career.
"She's tough and genuine and has danced every dance. She came out and finished fourth in the Guineas, second in the Oaks, second in the Ribblesdale, won the Nassau and then heads on to today. We came into today very confident.
"The plan was always to come here after the Nassau and try and put this one in the back of the net and then head on to the Breeders' Cup Filly And Mare Turf.
"That's the plan - hopefully we'll head to the Breeders' Cup."
O'Brien said of Magic Wand: "She ran well, I'm delighted with her and she has options.
"But it's a funny time of year for the fillies, so let's get her back and see how she comes out of today."