Hewick's rider Gavin Sheehan admitted that if it had not "been the King George I would have pulled him up" as his mount - purchased for just 850 euros (£738) and trained in Ireland by John 'Shark' Hanlon - trailed the other five runners for the majority of the race.
Shishkin ran anything like the "plonker" his venerable trainer Nicky Henderson had described him as last week due to his refusing to race on his seasonal debut.
He took up the running from 2020 King George winner Frodon six fences out and looked in control when he jumped the second last.
However, three paces on he stumbled and his jockey Nico de Boinville crashed to the ground.
"I am very proud of the horse," said de Boinville.
"These things happen. Racing can be cruel.
"I have never had a roar like that from a crowd after a fall, so fairplay to them."
His mishap appeared to leave the race in the hands of last year's winner Bravemansgame and favourite Allaho.
Allaho touched down in front after the last fence but out of nowhere Sheehan produced US Grand National winner Hewick, who brooked no argument as he eased past both of the horses in front of him.
'Biggest heart'
Sheehan stood up in the saddle and saluted the crowd as he crossed the line with Bravemansgame edging Allaho for second.
Sheehan - who picked up the ride as regular jockey Gainford is injured - said he had thought it was pointless as Hewick was all but tailed off going out on the second circuit.
"Put it this way if it had not been a King George I would have pulled up," said Sheehan.
"He stays and stays but there are only a certain amount of questions you can ask him.
"'Shark' said to me when he asked me to ride him that he is probably the smallest horse in the race but he has the biggest heart.
"This is massive. I am absolutely delighted, I am gobsmacked as a King George is not an easy one to win."
For larger-than-life character Hanlon it was not at all a case of desperation as he watched his horse struggling for most of the race.
"The ride Gavin gave him is unreal," said Hanlon, whose nickname 'Shark' has stuck since being called that by a selector at his hurling club in Kilkenny when he was 13 due to being far bigger than his fellow teammates.
"Hewick will stay all day but I think a tougher track would be a better job for him.
"We knew coming here there were three to four front runners who would set a fierce pace.
"We will now go straight to Cheltenham in March, we probably should go for the English Grand National but there is only one Gold Cup."
