Happily lunged late to beat the boys in the Qatar Prix Jean-Luc Lagardere and earn quotes from Sky Bet of 6/1 (from 8s) for the QIPCO 1000 Guineas and 7/1 (from 10s) for the Investec Oaks.
Aidan O'Brien's filly was bidding for a second successive Group One success following her narrow victory over stablemate Magical in the Moyglare Stud Stakes at the Curragh three weeks ago.
With Magical the chosen Ballydoyle representative in the Prix Marcel Boussac - finishing fourth - Happily instead took on the colts and was a hot favourite under Ryan Moore.
Charlie Appleby's pair of Mythical Magic and Masar controlled the pace, with leading home-trained runner Olmedo splitting the pair at the top of the home straight.
Masar responded to pressure to stay with the strong-travelling Olmedo, but neither had any answer when Happily quickened by in the last half-furlong.
Olmedo beat Masar to the runner-up spot, with Mythical Magic fading into fifth place.
Moore said: "She was already a Group One winner over seven furlongs, so we were confident she would get the trip well. We started racing a long way from home and she showed a great will to win.
"For a filly to beat the colts is a good thing for the future."
O'Brien said: "It was the lads who decided to run her against the colts, so it was a brave call. She's a tough, hardy filly and is a sister to Gleneagles. Ryan gave her brilliant ride. The fillies' allowance wasn't a disadvantage and she won well on the line.
"She showed she got the mile well so we think the (Juvenile) Fillies' race (Breeders' Cup) could suit her."
Appleby said of his two runners: "It doesn't look like Mythical Magic stayed and I think we'll bring him back to seven furlongs for his next run.
"I think Masar will be a better horse on better ground. He's got bags of ability, but that will be it for this year. James (Doyle) said he didn't take the bend very well."
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