Harryson, ranked 114th in the world, is now through to the Round of 16, where he will face world number five Jonny Clayton on Tuesday for a place in the quarter-finals.
Upon beating Germany's Pietreczko to take his tournament prize money to £60,000, an emotional "Dirty Harry" tried to put his achievement into words in the post-match press conference.
"I'm overjoyed. It's fantastic, I couldn't have imagined this," he said.
Yet despite getting the better of a player ranked 81 places above him, the Swede still believes there is room for improvement: "I think I could have played better," he said.
The match with Welshman Clayton will require the Malilla man to be at his very best to cause another upset, though he could be spurred on by the prospect of receiving a tour card should he reach the last eight, effectively allowing him to turn professional.
But for now, Harryson says he is simply focused on enjoying the showdown with Clayton: "I'm really looking forward to the match, it's going to be awesome."