Djakadam travelled well for most of the race and looked as though he might end Willie Mullins' hoodoo in the showpiece event, but Sizing John (7-1) pounced to lead between the final two fences.

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).

Native River (7-2) was a short-head away in third and Djakadam - twice the runner-up - was fourth.

Owner Alan Potts said: "It's unreal, it's my dream and it's come true.

"It's our first runner in the race, the jockey's first ride and Jessie's first runner."

Harrington said: "I can't believe it, it was amazing how he jumped and travelled.

"Robert said he wasn't going to go down the inside, but he did.

"It's absolutely fantastic, my first runner in a Gold Cup.

"He was running over two miles, but it was Robert who said he would stay and he did.

"Thanks to everyone at home, it's all down to the team.

"I can't believe it, I've got to get a Classic now (on the Flat)!"

Cue Card fell at the third-last for the second successive year, while Lizzie Kelly only got as far as the second fence where she and Tea For Two parted company.

Power, who won the Grand National on the Gordon Elliott-trained Silver Birch in 2007, said: "Unbelievable, Jessica Harrington is a genius.

"I always said when this horse stepped up in trip he'd win.

"It's 10 years ago I won the National, I'm 35 now and appreciate it a bit more.

"Everyone has done a top-class job, it means more than any winner I've ridden for Jessica."