Just over four months after recording his first winner as a trainer, 21-year-old Donnacha O'Brien has won the Prix de Diane at Chantilly with Fancy Blue.
Jessica Harrington's Alpine Star was second and Aidan O'Brien's Peaceful was third in a 1-2-3 for Irish-trained horses.
The former dual champion jockey only took out his training licence earlier this year, following in the footsteps of his father Aidan, who completed the Derby and Oaks double at Epsom on Saturday, and brother Joseph.
Fancy Blue came into the race having filled the runner-up spot behind O'Brien senior's Peaceful in the Irish 1,000 Guineas three weeks ago.
The pair renewed rivalry in France and were joined by Alpine Star, who was sent off favourite to follow up her triumph in the Coronation Stakes at Royal Ascot.
Drawn one, the latter made much of the running in the hands of Stephane Pasquier and did not go down without a fight, but Fancy Blue eventually wore her down under Pierre-Charles Boudot to claim a narrow victory.
Peaceful was just behind her compatriots in third.
The winner returned 81-10 on the French-based PMU, with an industry SP of 13-2.
#Plat #Chantilly #DianeJockeyAtHome @Longines— Equidia (@equidia) July 5, 2020
La fille de #DeepImpact, Fancy Blue 🇮🇪 (Declan O'Brien), remporte l'édition 2020 du Prix de Diane Longines. La favorite, Alpine Star 🇮🇪 (Jessica Harrington) est deuxième, Peaceful 🇮🇪 (A. P. O'Brien) termine troisième. pic.twitter.com/nqTTKJjqOt
"There's so much pressure and a lot of work goes into it, but this feeling is great and thank God she won," the triumphant trainer said of what looks to be the first of many big-race successes.
"I always knew she was a very good filly and we always thought the step up in trip would suit her. It definitely did.
"Pierre is a very good rider and I told him to do what he wants. There was no pace early so he went forward on her and sat upsides the leader. It was exactly what I would have liked him to do. She quickened up well and stuck at it. Fair play to Pierre, he gave her a peach of a ride.
"The home straight felt like a lifetime, I didn't really know what was going on. Thankfully, at the end of it all, she got her head in front.
"I was glad we didn't go to Epsom after what I saw yesterday. I would say Love is a fair machine. Thankfully, we made the right call in sending her to France.
"We'll get her home and see how she is. We might have a look at the Irish Oaks at the Curragh."
He added: "I've really enjoyed the training so far and if I keep getting horses who can run in big races like the French Oaks I will keep on enjoying it! This means a lot. The lads at home are the ones putting in the work and I'm delighted for them more than anything."