William Haggas scored a memorable Group Three double with his fillies Monica Sheriff and Miss O Connor at Saint-Cloud.
Haggas was not part of the strong British challenge soundly repelled as Pia Brandt's Mkfancy kept the Group One One Criterium de Saint-Cloud at home.
But either side of that staying feature for top-class juveniles on Saturday's card, the Newmarket trainer's two runners each revelled in very testing conditions to win decisively.
First, Monica Sheriff justified short odds in the Prix Belle de Nuit as Tom Marquand won his first race in France, and then Miss O Connor was a successful favourite too in the Prix Perth.
Haggas' wife and assistant trainer Maureen, on hand to witness the yard's perfect day, said: "It's been brilliant, great.
"They liked the ground, and they haven't had much of it this year."
The Criterium de Saint-Cloud is a historic race won by many future champions - including this year's Prix de l'Arc de Triomphe hero Waldgeist, in 2016.
Previously the winner of a mile maiden at this course on his second career start, Brandt's colt Mkfancy led throughout over two furlongs further and comfortably kept the travellers at bay.
The 8-1 winner had three lengths to spare at the line, under Theo Bachelot, chased home by two of Aidan O'Brien's three contenders as Arthur's Kingdom stayed on again late in second and Mythical held on for third.
Brandt told Sky Sports Racing: "He's home-made, more or less - the breeder still owns him.
"They are all here, so it is great.
"We thought it was going to be a faster pace, but he did all the work himself.
"Theo had confidence in the horse and let him go off in front - and he went all the way to the line."
The Haggas stable ensured the travelling contingent fared much better in two of the card's three Group Threes.
Monica Sheriff took her winning streak to five, tackling Pattern company for the first time following her 20-length demolition of a handicap field at Goodwood in similarly heavy ground a month ago.
The three-year-old won by a hard-earned but ultimately decisive length and a three-quarters, from Endorphine.
Marquand was prominent early alongside fellow British hope Vivid Diamond, before settling in third and then overhauling Endorphine two furlongs out and staying on well.
He said: "The ground wasn't really a worry with her - obviously, she handled the horrific ground at Goodwood, and she certainly stays all day too.
"She's really coming together mentally and physically. We had to swing wide round the outside of them, and she had to use a little bit of toe to just hold her position - and she did so nicely, and fair play to her, she stayed on really well.
"This is my first ever winner in France, so it's a nice way to start off."
Miss O Connor completed the stable's double, leading throughout under Pierre-Charles Boudot to win by a length and extend her unbeaten career tally to four.
Haggas said: "She did it really nicely - I thought she was beat, I thought he'd gone quite quick.
"She's probably a bit fresh, because she's been off for so long (since August), but she's really tough. I was impressed with her.
"She could have just said 'OK' - but she really wanted to win, which was great, and I'm thrilled.
"Our son bought this filly. William wasn't sure he was doing the right thing, but she's done really well - and she's talented. She's done nothing but improve, and she does love this ground."