Aidan O'Brien's stayer was sent off the 4/5 market leader, but with two furlongs to run he looked beaten back in fourth, hitting a big price on the exchanges as Torcedor kicked clear.
Gold Cup hero Big Orange was a spent force a long way out on the testing ground and it was Mount Moriah who led them into the straight, with Torcedor on his shoulder.
When Colm O'Donoghue asked the Jessica Harrington-trained Torcedor for maximum effort he looked sure to collect, but in the final 100 yards he was treading water as Ryan Moore got a late lunge out of Order Of St George.
John Gosden's Goodwood Cup winner Stradivarius stayed on for third under Frankie Dettori.
Moore told ITV Racing: "I was very happy the whole way round and thought I had the first two covered, but they picked up very well and it didn't look likely for a long time. But as they came back to him he began to get motivated and at the line he's won well.
"He's an unbelievably good horse, he ran a very good race in the Arc 20 days ago, he's come here at the end of a long, hard year and looked magnificent.
"He's a pleasure to deal with and we're very lucky to have him."
O'Brien said of his five-year-old, who was fourth to Enable in the Prix de l'Arc de Triomphe: "He is tough and hardy and Ryan gave him a great ride. He never stops. In every race he has ever run in, he is always finishing.
"We saw him in the Gold Cup. He was finishing. Sometimes the line comes too quick, but he doesn't stop. He has had a busy time. Obviously we are delighted with him. That is him for this year, I imagine."
Harrington said: "He's run a real cracker, I'm absolutely thrilled. He's been beaten by a very good horse, it's as simple as that.
"He put it up to him and he's come on again since the summer and I've only had him a year, not even a year in training. I'm still learning (about him) and he'll be back next year."
Gosden said: "He's run a blinder. It's not his favourite ground, he has this ability to quicken and you can't really quicken in that.
"It was a tremendous race, Order Of St George brings Arc form in here. Jessie's horse looked like he'd got it. We're thrilled with him and he's a better horse on good ground.
"He stays in training and we look forward to races like the Ascot Gold Cup."
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