Sir Henry Cecil's Twice Over beat stablemate Midday to win the Juddmonte International Stakes at York.
Twice Over (11-2), ridden by Ian Mongan, won by three quarters of a length with favourite Await The Dawn third.
Earlier Sea Moon was made hot favourite for the St Leger at Doncaster after winning the Great Voltigeur Stakes.
The 11-2 shot, trained by Sir Michael Stoute and ridden by Richard Hughes, won by eight lengths.
Mongan was riding dual champion Stakes winner Twice Over for the first time as owner Khalid Abdulla, whose Juddmonte operation bred the first three home, celebrated his maiden victory in a race he has sponsored for 22 years.
On the opening day of York's extended four-day Ebor Festival, he edged out Cecil's stable jockey Tom Queally on the six-time Group One-winning mare Midday.
Mongan, securing the biggest win of his career, said: "He's a true champion, this horse, and really stuck his neck out."
Cecil paid tribute to the winning jockey: "I've always thought a lot of him. He's my second jockey and is very underrated."
In the Great Voltigeur, odds-on favourite Seville was well placed with a furlong to go, but Sea Moon soon pulled clear.
Al Kazeem stayed on for second, with Seville passing the post a disappointing third.
Sea Moon was made 6-4 favourites by sponsors Ladbrokes for the St Leger at Doncaster on 10 September.
Hughes said: "He's a big, raw horse, but is an exceptional mover."