Given a patient ride by Robbie Power, the seven-year-old completed the English-Irish Gold Cup double in style to dispel any stamina doubts.
Djakadam's jockey Ruby Walsh, who rode four winners on Thursday, said it hadn't cost him the race just the same spot he had filled in the previous two editions.
So often in the shadow of Douvan over two miles, Sizing John kept on strongly up the hill to score by two and three-quarter lengths from the staying-on Minella Rocco (18-1).
Under Power's flawless stalking ride, Sizing John had both Djakadam and Native River in his sights at the second last. It's a dream come true. "He jumped like a buck and it's his jumping that got him there the whole way", she said.
"Robert said he wasn't going to go down the inside, but he did".
At this stage you're probably looking for something to have a small flutter on before the Gold Cup so why not L'Ami Serge (20/1) in the County Hurdle (2.10pm) who has 12 top three finishes in his 13 runs. It is run over a distance of three miles, two and a half furlongs and they will jump a total of 22 fences.
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"Thanks to everyone at home, it's all down to the team". Champagne Classic was described rather gracelessly by Michael O'Leary as "the worst horse I own", but he will have a special place in the heart of young jockey JJ Slevin, a cousin of Joseph O'Brien, who secured his first Festival victor.
There was early disappointment too for Lizzie Kelly - only the second female jockey to ride in the Gold Cup after Linda Sheedy in 1984 - as she was unseated by Tea For Two at the second fence, slamming her fist into the turf in frustration.
10 years ago Power, the son of Captain Con Power, a former Irish showjumper, won the Grand National at Aintree on Silver Birch.
"I still can't believe it", said Harrington.
"I have never had a runner in the race so to train the victor, I don't know when I am going to come back to earth", said the County Kildare-based trainer, wearing a cast on her left arm after a skiing accident. Everyone has done a top-class job. But I had a lot of confidence in this horse.
"We upped him to two and a half and then up to three - I was fully confident he'd always stay". Willie Mullins' Djakadam is the current market leader and has been the most popular selection on site with 27% of users in the last 48 hours putting their money on the 8yo to give his illustrious trainer a first win in the race.