Study Of Man claimed Classic glory in the QIPCO Prix du Jockey Club at Chantilly.
Pascal Bary's colt was among the leading contenders following an impressive display in the Prix Greffulhe at Saint-Cloud last month and was always travelling strongly in midfield.
He began to reel in the leaders early in the home straight under Stephane Pasquier and battled his way to the lead inside the final furlong.
Several horses attempted to peg him back as the post loomed, but Study Of Man, a son of Deep Impact, clung on for victory.
James Tate's French 2000 Guineas runner-up Hey Gaman looked set to play a significant part for a long way before fading late in the piece. His Poule d'Essai des Poulains conqueror Olmedo, however, was bitterly disappointing.
Key Victory came up short in his bid to provide trainer Charlie Appleby, jockey William Buick and Sheikh Mohammed's Godolphin operation with a second Derby success in the space of 24 hours following Masar's triumph at Epsom on Saturday.
Aidan O'Brien saddled a quartet of runners in Flag Of Honour, Hunting Horn, Kenya and Rostropovich, but all finished unplaced.
Sky Bet cut the winner to 10/1 from 16/1 for the Prix de l'Arc de Triomphe.
Alan Cooper, racing manager to winning owners the Niarchos family, said: "He did it very well and he's shown what Pascal and Stephane have always thought of him was correct - he's a top-class horse.
"After he won the Prix Greffulhe it was either come here or go to Epsom (for the Investec Derby) and we decided to stay at this distance (mile and a quarter) for the moment.
"We'll regroup now and see whether he could be a horse for the Prix de l'Arc de Triomphe, but let's not jump to any conclusions.
"We'll get him back and get ourselves back and go from there."
Andre Fabre claimed his 10th Grand Prix de Chantilly with the hugely impressive Waldgeist.
The son of Galileo had failed to realise his full potential since finishing fourth in last summer's Irish Derby, but landed a Group Three at Longchamp last month to end a losing run dating back to 2016.
Stepping up to Group Two level for his latest assignment, Waldgeist travelled well in behind Charlie Appleby's pacesetter Rare Rhythm before easing to the lead early in the home straight.
Stable companion Cloth Of Stars did not get the best of runs, but would have struggled to lay a glove on Waldgeist in any case as he romped to a wide-margin victory in the hands of Pierre-Charles Boudot.
British challengers were out of luck as Mission Impassible claimed top honours in Prix de Sandringham at Chantilly.
Roger Varian's Altyn Orda appeared a major contender for the Group Two prize off the back of finishing fifth in last month's 1000 Guineas at Newmarket and was joined on the trip across the Channel by the Karl Burke-trained Ellthea, who placed seventh in the French Guineas.
Freddy Head's Efaadah cut out much of the running, with Altyn Orda and Ellthea never too far off the pace.
However, the raiders faltered in the home straight and it was Jean-Claude Rouget's Mission Impassible who came home best to score a shade comfortably under Cristian Demuro.
Crown Walk finished well to beat Efaadah to the runner-up spot.
Ellthea and Altyn Orda passed the post in fifth and sixth place respectively.
Rouget said: "She was a lot calmer than in her previous two races. We wanted to be clear in our minds by giving her another chance.
"At present, I'm not thinking of going to Royal Ascot with her as it will come too soon. Ditto the Prix de Diane. We are going to train her with a view to going to Deauville and why not have a look at the British programme for fillies?"
Finsbury Square was an emphatic winner of the Prix du Gros-Chene for trainer Mauricio Delcher-Sanchez and jockey Olivier Peslier.
Charlie Hills sent last month's Chester scorer A Momentofmadness across the water for the Group Two prize, but after forcing the pace he eventually weakened and passed the post third.
Finsbury Square was ridden with more restraint by Peslier and powered clear late on to score with plenty in hand from Gold Vibe.
Peslier completed a double on the day aboard Pollara in the Prix de Royaumont.
Francis-Henri Graffard's inmate squeezed through a gap between two rivals on the far side of the track and came home strongly once in the clear to win with plenty in hand.
Varian's Shaherezada was well-beaten.