Clan Des Obeaux and Harry Cobden finished strongly to secure a 10th King George VI Chase win for trainer Paul Nicholls at Kempton.
The horse, part owned by Sir Alex Ferguson, battled with veteran Thistlecrack from the home turn.
But a strong jump from the 12-1 shot at the last fence saw him pull clear of his rival.
He powered on to win by a length and a half with Gold Cup winner Native River back in third.
Clan des Obeaux had looked less than impressive in his last outing at the Betfair Chase at Haydock but at only six years of age, Nicholls was always confident he would get better with more experience.
Former Gold Cup winner Coneygree had shown plenty of spirit early on with Might Bite also prominent and Thistlecrack keeping in touch with the leaders too.
There was drama as they embarked on the second circuit when Bristol De Mai parted company with Daryl Jacob and brought down the Ruth Jefferson-trained Waiting Patiently and jockey Brian Hughes in the process.
Thistlecrack looked strong as he took the lead but Cobden had a patient ride on Clan Des Obeaux, four years younger than his rival, and the pair showed their strength when it mattered to claim a famous win.
Cobden, who recently turned 20, was named as Nicholls's stable jockey this season in succession to Sam Twiston-Davies and had opted to ride the winner rather than stablemate Politologue.
"I've grown up watching the King George," he said afterwards. "I never thought I would be in a position to ride a winner in it.
"But he has improved a lot for his last run and he has done it really well in the end.
"I definitely thought he would improve. He is very lazy at home and didn't do much from the last. I was struggling to keep up most of the way, but his jumping put him in there."
Nicholls added: "10 King George wins is amazing. If you look at the form he didn't have much to make up on Thistlecrack. We've always believed in him a lot, as you know.
"It was a very good run at Haydock for the stage of his career and he sort of got forgotten. Some said he was not good enough, but horses progress. He was like a footballer coming from the Championship going into the Premier League that day.
"He got a little bit lost, but he stayed on and it was the basis of a good future and I think there is a lot to come from him. To win races like this means more than anything, I can tell you. He is a proper Gold Cup contender."
Earlier, Verdana Blue ended the long unbeaten run of Nicky Henderson-trained stablemate and dual champion hurdler Buveur D'Air in the Christmas Hurdle.
Buveur D'Air, ridden by Barry Geraghty, who went off as the 1-4 favourite, looked to have recovered from a bad mistake at the third last.
But Verdana Blue and Nico de Boinville never gave up and won by a short head to give Henderson an eighth win in the race.
Cornelius Lysaght, BBC racing correspondent
Paul Nicholls was champion jumps trainer every season bar one from 2005 until 2016, his prize money totals regularly boosted by King George success.
Since losing the title, he's regularly spoken of a "rebuilding" process at his Ditcheat, Somerset stables, and it's horses like Clan Des Obeaux that will take him back to the top.
Given a cool ride well beyond his 20 years by Harry Cobden, the horse comfortably accounted for some of the best chasers around and looks Gold Cup material.
The other to take out was Native River - on a course too flat and over a distance too short, he was still third.