Lucas Bjerregaard fired a final round of 67 to overhaul defending champion Tyrrell Hatton and win the Alfred Dunhill Links Championship.
Hatton looked in a strong position to win the tournament for a third straight year after back-to-back scores of 66 left him in the lead heading into Sunday's play at St Andrews.
The Englishman - fresh from helping Europe win the Ryder Cup in France - reeled off four successive birdies on his front nine to remain in control, only to suffer a surprising collapse after the turn.
He had back-to-back bogeys at the 10th and 11th before dropping further shots at the 15th and 16th, leaving him to sign for a level-par 72 and a share of second place with Ryder Cup team-mate Tommy Fleetwood.
Bjerregaard, who had trailed by as many as six shots with 12 holes to play, finished one ahead of the big-name duo to secure his second European Tour title and move into the top 10 in the Race to Dubai rankings.
"It didn't really look like I was in it today - Tyrrell looked like he was going to take off with it," the delighted Dane told Sky Sports.
"I'm really happy now. It's been a great year, but now it's got a whole lot better."
The 27-year-old crucially bounced back from bogeying the penultimate hole with a three at the par-four last, with Hatton - bidding to become the fifth player to win the same event in three consecutive seasons - unable to force a play-off when he failed to find the cup with a birdie putt of his own on the 18th green.
"I guess my putt on the last to get in the play-off sums it up, really. Just a massive gust of wind knocks me off, pretty much lose balance on a putt, which is unfortunate," a dejected Hatton told the European Tour's website.
"I'm pretty disappointed because I had a five-shot lead at one point. It was in my hands."
Fleetwood had set the clubhouse target at 14 under by signing for a 69, though it could have even better for the Englishman had he taken a birdie opportunity on the final hole.
Tapio Pulkkanen of Finland was alone in fourth on 12 under while American pair Brooks Koepka and Tony Finau both finished inside the top 10, ending on nine and eight under respectively.
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