Star filly Treve lost her unbeaten record after a thrilling tussle with Cirrus des Aigles in the Prix Ganay at Longchamp.
In a high-class renewal of the contest, the field were taken along at a fair pace by the favourite's stablemate Belle De Crecy from the start.
Ed Dunlop's Joshua Tree took up the advantage in the false straight, coming wide off the bend along with the pacemaker, but against the far rail the real race was developing with Christophe Soumillon sending Cirrus Des Aigles to the front and firing him up.
Frankie Dettori brought Treve on his outside two furlongs out and there was little between the pair from that point onwards, Treve narrowly edging ahead, before Cirrus Des Aigles (returned the 7/2 second-favourite with British bookmakers and 100/30 on the Pari-Mutuel) battled back to regain the advantage in the final strides and score by a short neck. Norse King finished four and a half lengths back in a never-nearer third.
The pair could clash again in the Prince of Wales' Stakes at Royal Ascot next month, although connections of the winner are looking first to Epsom. Bookmakers Sky Bet make Treve 6/4 from 11/10 for the Prince Of Wales's Stakes, with Cirrus Des Aigles in from 9/1 to 9/2.
Corinne Barande-Barbe, the ever-enthusiastic trainer of the winning eight-year-old, felt that the smart early pace had suited her charge and is next eyeing a trip to Epsom for the Investec Coronation Cup on June 7.
She said: "I'm very proud of what he did today - he never was better. I thought the strong pace helped us and it was obviously one of his finest moments.
"On reflection, if the there had been a stronger pace in the race against Frankel (2012 Champion Stakes at Ascot), we might have made that a bit more interesting but now all roads will lead to the Coronation Cup."
Soumillon hailed Cirrus Des Aigles, who was conceding 3lb to Treve, as "perfect" and believes the gelding was showing his best form.
He said: "I'm very, very happy. Although it's the first Group One of the season, it's not the biggest race but then again, the match was built up like a big game and my horse was perfect.
"He had his track, his ground and two races under his belt - it made sure he would operate to the maximum - but this is really something extraordinary for racing.
"It's what we love in European sport - horses so closely matched and fighting it out together. He is probably on his top level of form and to beat Treve, he had to be there."
Treve's trainer Criquette Head-Maarek felt the easy ground might have gone against her charge and said that she remained confident of avenging the defeat should the pair clash again this term.
She said: "Maybe there was too much rain, it was sticky out there and we had too much ground to make up. It was a very good run but the only thing I will say is that my pacemaker (Belle De Crecy) went too fast.
"The winner had had two races already and the fitness was there.
"If she comes out of it well, in principal we will be going to Royal Ascot. We might have lost a battle, but we haven't lost a war.
"She is brave but the other horse is stronger with those two runs this year and he loves that ground. She ran a grat race but he was fitter than we were.
"In the rematch, I'm sure we can get it back. She took the lead but in the end he's come back at us - that's racing. Horses are not unbeatable, that is for certain."
Harry Herbert, racing manager for Sheikh Joaan al Thani's Al Shaqab Racing, also insisted that the Prince Of Wales's Stakes at Royal Ascot in June remains very much on the agenda.
He said: "It's frustrating in that she went to the front and you could see it was going to be a battle. The winner's form was very strong and he was always going to be a big danger.
"The fitness deal with your first run on this ground means so much and on this going she doesn't have that zippity-zip.
"It's all very much to play for in the bigger picture and we will be heading to Royal Ascot as planned."
Soumillon had earlier earned another hard-fought victory as Vazira kept her unbeaten record intact in the Prix Vanteaux.
Alain de Royer-Dupre's Aga Khan-owned filly was an 8-11 favourite for the Group Three prize having made a winning debut at Chantilly at the end of March.
Odds-on backers were made to sweat as Soumillon was riding along vigorously over a furlong down, but his mount responded well to fend off the challenge of Kenzadargent.
Vazira, a daughter of Sea The Stars, holds entries in the Coronation Stakes at Royal Ascot and the Darley Irish Oaks at the Curragh.
The Andre Fabre-trained Montclair claimed the glory in the other Group Three prize of the afternoon, landing the Prix de Barbeville from Terrubi.