The Welshman, ranked world number five, was made to earn every inch by a Swede playing with freedom and audacity.
Clayton edged the opening set 3-2, recovering from early pressure and a missed bull in the decider before tidying 39 to settle initial nerves.
Harrysson immediately answered back in set two, producing two outstanding ton-plus finishes, including a sublime 124 and a clinical 149, to level the match at 1-1 and announce that this was no fleeting cameo.
Set three tilted the contest towards the underdog. Harrysson dismantled 96 in 14 darts, survived a moment of jeopardy after Clayton detonated a 160 checkout, then calmly executed an 11-dart decider to move 2-1 ahead. At that stage, the Swede looked sharper, braver and entirely untroubled by the surroundings.
Clayton, though, began to wrestle back control in the fourth. While Harrysson continued to punish loose visits, the Welshman tightened on the outer ring, pinning double 16 in the decider to square the match at 2-2.
That pressure exploded in set five as Harrysson broke throw twice and stood on the brink of a commanding lead, only to miss a clutch of set darts at double 18 and double nine.
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Clayton seized the reprieve, forced a decider and then stole the set on tops, roaring in celebration as the crowd sensed the emotional swing.
With both players averaging around the 93 mark and trading maximums evenly, the deciding set was fittingly dramatic.
Clayton struck first with a composed 72 checkout, only for Harrysson to respond with six perfect darts via an unconventional route that briefly teased a nine-darter before the bull denied him. Once again, neither man blinked.
The match finally turned on composure. Harrysson missed opportunities at double 20 and later double four, and Clayton, under siege but unbroken, found double eight to stay alive.
In the decider, with the darts in hand, Clayton delivered a timely maximum, refused the bull from 121 and calmly set up his favoured double. One clean dart at double 16 sealed a 3-2 set and a 4-2 match victory.
'That was tough'
"There are a lot of things on my mind, obviously moving to world number four was one of them," Clayton told Sky Sports afterwards.
"To be fair to Andreas, every time I looked up, he was hitting doubles.
"That was tough. I didn't play my best, there were a lot of loose darts, but that's what pressure does to you. And that's what Andreas did, kept me under pressure the whole time."
Clayton - now provisionally ranked fourth in the world - moves on to face Ryan Searle, fully aware that standards will need to rise.
Happy Feet Hood humbles Rock
New fan favourite Justin Hood produced one of the performances of the tournament to sweep aside Josh Rock 4-0 to round off the afternoon, delivering a masterclass in pressure finishing that left one of the sport's brightest talents stunned.
From the outset, Hood was unrelenting. He broke immediately with a crisp 108 checkout and closed the opening set with an 11-darter, refusing to miss a double as Rock was forced into a reactive role. Rock briefly broke through in the second set, but Hood snapped straight back, stayed flawless on the outer ring and sealed another deciding leg on double top to surge 2-0 ahead.
Hood continued to score heavily in the third set and simply would not miss, reeling off finishes on double top and double four before tearing through another 11-darter. Nine darts at doubles, nine hits. Rock could only watch as the deficit grew.
The fourth set bordered on surreal. Hood opened with yet another 11-darter, stretching his run to an astonishing 10 from 10 on the doubles, then broke again to move one leg from victory.
The streak finally cracked as four match darts went begging, allowing Rock a brief reprieve. It changed nothing. Hood regrouped instantly, dismantled 119 and pinned double 13 first dart to complete a remarkable 4-0 win.
Hood hit 10 maximums to Rock's three, landed two ton-plus checkouts and still finished with a formidable 75 per cent success rate on the doubles, having been 11 from 11 at his peak. It was clinical, fearless and utterly decisive.
He now advances to a quarter-final against either Michael van Gerwen or Gary Anderson. For Rock, this was a chastening exit, undone by an opponent operating at a level few could live with.
Ratajski sets up Littler clash
Krzysztof Ratajski opened the day with a controlled 4-2 victory over Luke Woodhouse, setting up a quarter-final meeting with reigning champion Luke Littler.
In a match dominated by breaks of throw, Ratajski's superior composure on the doubles proved decisive. The opening set was scrappy, but Woodhouse's missed darts at double 16 were punished as Ratajski claimed it 3-1 while averaging over 106.
Woodhouse hit back in set two, breaking with a tidy 44 checkout and producing the finish of the match, a precise 110, to draw level. That momentum did not last.
In set three, Woodhouse thrilled the crowd with eight perfect darts before missing double 12 for a nine-darter and then busting, allowing Ratajski to regain control and move 2-1 ahead.
Woodhouse showed resilience to square the match again in set four, breaking early and sealing the decider with authority on double 16. The fifth set turned on fine margins.
Despite opening strongly, Woodhouse was undone by costly bounce-outs and a missed dart at double 20, as Ratajski punished ruthlessly to edge it 3-2.
Ratajski closed the contest efficiently in the sixth, controlling the scoring and tidying up on double 16 to complete a disciplined and decisive performance.
The fourth round action rumbles on into Tuesday evening, with a stellar lineup including Luke Humphries, Michael van Gerwen, Gian van Veen and Gary Anderson all vying for a quarter-final spot.
Tuesday afternoon sessions:
Luke Woodhouse 2-4 Krzysztof Ratajski
Jonny Clayton 4-2 Andreas Harrysson