Joe Fanning was at his brilliant best as Subjectivist took Stradivarius' Gold Cup crown.
Punters expected the race favourite to win his fourth renewal on the trot, but Frankie Dettori endured a nightmare passage in the Royal Ascot feature.
Caught in behind several rivals, Stradivarius could not get out at a crucial stage and by that time Subjectivist had stolen a winning advantage.
Confident in his mount's stamina for the 2m 4f trip, Fanning sent the winner for home early enough and they kept up the gallop for a five-length victory.
Princess Zoe (28-1) took second and Spanish Mission (7-1) was third. Stradivarius was just behind them in fourth.
Fanning was congratulated by his weighing room colleagues returning to the winners' enclosure aboard the Mark Johnston-trained winner.
"I was (always confident in his stamina) to be fair," he said.
"I never felt that was an issue. He can go on any ground you know.
"Oh great, it's great for the yard and Dr Jim a brilliant owner for us for years."
Dr Jim Walker added: "It's nice to be in the winning enclosure at Ascot for once! It's the pinnacle. Fantastic."
Johnston, who last won the Gold Cup in 2002 when Royal Rebel doubled up, said it was a "sad day" to see Stradivarius beaten.
He congratulated Fanning for his horsemanship aboard the 13-2 winner.
"I know what it feels like when a champion goes under but it had to happen one day," he added.
"I think it's his third Group One Joe, he deserves it. We really saw him at his best there. I don't think there's anybody better than Joe Fanning, he's absolutely perfect at setting the pace."
Subjectivist was bred by Eddie Hearn's mum Susan at Mascalls Stud.
To date the horse has earned connections £733,536.
