Enable became the first horse to regain the King George VI and Queen Elizabeth Stakes crown at Ascot as she edged out Crystal Ocean in an epic duel.
The 8-15 favourite, ridden by Frankie Dettori for trainer John Gosden, went head to head with the runner-up for two furlongs of a classic racing battle.
Crystal Ocean, the Sir Michael Stoute runner ridden by James Doyle, briefly headed Enable but she fought back.
Waldgeist was third as 2017 winner Enable clocked up an 11th straight win.
Delighted Dettori had called his mount "the darling of racing" before the mile-and-a-half contest, worth £1.25m in prize money, and said to her after victory by a neck: "I love you baby."
"I'm exhausted, emotionally exhausted too. I'm lost for words. She is loved by everyone," said Dettori as the five-year-old mare enjoyed a rapturous reception after a ninth consecutive Group One triumph.
"She's brilliant. The second [Crystal Ocean] is a great horse, I knew he was the one to beat. We gave the public what they came to see, and they got a fantastic horse race."
The 48-year-old rider's magnificent run of form has prompted some bookmakers to restrict accumulator bets on the Italian-born jockey after he almost cost them a fortune when narrowly missing out on riding five winners in one day at Royal Ascot.
Dettori, who has won more than 3,000 races in his career, said the 2019 King George was the toughest top-level contest he had experienced and that Crystal Ocean - who was also second last year, to Poet's Word - was "like one of those flies that wasn't going away".
'Brave and wonderful' - story of the race
Enable found herself wider than ideal through the early stages as Norway - one of four contenders trained by Aidan O'Brien - made the running.
After O'Brien's Derby winner Anthony Van Dyck dropped away, Enable and Crystal Ocean went to the front in the home straight.
As they passed the two-furlong marker, with a quarter of a mile to go, battle commenced and a bumper crowd witnessed a tussle that bore echoes of the Grundy-Bustino showdown of 1975.
Enable prevailed and remains on track to aim for an unprecedented third success in Europe's richest race, the Prix de l'Arc de Triomphe at Longchamp, in October.
Gosden, who indicated York's Ebor meeting next month remained on the agenda as her next outing, said: "She's a brave and wonderful filly, and what a great battle with Crystal Ocean."
'The best since Frankel' - what they said
Enable trainer John Gosden: "Crystal Ocean showed all the courage in the world, and the first two are wonderful - they put up a great King George."
Jockey Frankie Dettori: "In all my career it is probably the hardest and most hard-fought race I've ever had. It was two great champions. Give credit to Crystal Ocean, he put it out there for me and it was an amazing race."
Crystal Ocean trainer Sir Michael Stoute: "He made it an entertaining day for the crowd. He doesn't know how to give in. He is just so brave and gives everything."
Jockey James Doyle: "I've never experienced finishing second in a race and getting as loud a cheer as the winner. They really do love him and he wears his heart on his sleeve. What a run. It was a great race, it was unbelievable."
Oisin Murphy, the rider of sixth-placed Cheval Grand: "Enable is the best since Frankel."
Frankel, who was retired for a breeding career in 2012 after 14 consecutive victories, ran, like Enable, in the green, pink and white silks of owner Khalid Abdullah.
BBC racing correspondent Cornelius Lysaght
Move over Grundy and Bustino! That breathless duel, in the King George of 1975, is regularly referenced as the greatest flat racing finish of the modern era, but after this encounter that's a perfectly legitimate line to take.
They went at it nostril to nostril through fully a quarter of a mile, the horse actually the highest rated in the world - Crystal Ocean - putting it up to Enable and the mare showing her reserves of grit in a finish that is likely to be talked of for decades to come.
And full marks to the crowds here: the second received nearly as many cheers as the winner.
|Enable - in stats|
|Only the third two-time winner of the King George - after Dahlia (1973, 1974) and Swain (1997, 1998)||Enable is a daughter of Nathaniel and a granddaughter of the prolific stallion and 2001 King George winner Galileo|
|Winner of 13 of her 14 races - last defeat was in April 2017||Earns +£700,000 for winning King George - taking career prize money to £9.1m|