As Cheltenham roared, Alan Potts couldn't look.

Sizing John, his relatively unfancied horse who in December was still racing over two miles, was galloping down the home straight of a three-miler - but one of the biggest three-mile races of all.

A Gold Cup was his reward, holding off competition from Native River, another strong fancy from Colin Tizzard's stable and game competition to the end.

But, in the end, it was fairytale stuff for owner Potts, trainer and first-time Gold Cup winner Jessie Harrington, and everyone involved with Sizing John.

“It's unreal,” he said. “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... I can't believe it, it was amazing how he jumped and travelled.”

Tactically it had appeared a curious tactic to stick to the rail by jockey Robbie Power, but a fortuitous fall, Cue Card's disappearance and Native River failing to last the distance opened up a little hole.

“Robert [Power] said he wasn't going to go down the inside, but he did. It's absolutely fantastic, my first runner in a Gold Cup.

Sizing John jumps the final hurdle on his way to victory (Getty)

“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)!”

Power, who won the Grand National on the Gordon Elliott-trained Silver Birch in 2007, described the day as “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.

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

“It's just amazing. Sizing John has gone from two miles at Christmas to three miles here. It's his jumping that got him here.”

Pushed by surprise contender Minella Rocco, a 20/1 outsider, Sizing John showed little sign of stress as he strode into the winner's circle.

Majestic and calm, in contrast to his frenzied surroundings, it was the win that his connections had barely thought possible in November. “It's amazing, it really opens your eyes,” said Robbie Power, the jockey.