Tyrrell Hatton sealed the first English win at the Arnold Palmer Invitational and his maiden PGA Tour title with an eventful one-shot victory at Bay Hill.
His two-stroke overnight lead had gone in the first five holes after two bogeys on lightning-fast greens.
Hatton, 28, also had a double bogey in a two-over 74 but finished four under, one ahead of 2017 winner Marc Leishman.
Rory McIlroy, joint second at the start of the day, had two double bogeys in a 76 and was four back in fifth.
"It was such a tough day," said Hatton. "After the double on 11 I thought I'd played my way out of it because I didn't see the scoreboard.
"That putt at 18 - I have never been so nervous. I won't ever forget this week."
There were 44 players under par after Thursday's first round but only four by the end of the tournament.
Saturday's scoring average of 75.91 was the highest of any round at Bay Hill since 1983 - with only one player under par - and it was the first non-major not to have a round in the 60's since 2014.
The conditions were similarly challenging for Sunday's final round because of more swirling winds and rock-hard putting surfaces and Hatton dropped a shot on his opening hole after finding a fairway bunker.
Playing partner Leishman, two shots back alongside 2018 winner McIlroy overnight, saw his challenge checked by a double bogey at the third after he found water with his drive.
McIlroy holed a fine putt from the edge of the green to drop only one shot at the fifth but then thinned one out of a fairway bunker onto the rocks at the next and ran up a double bogey to drop two shots off the lead.
A drive out of bounds led to another double bogey at the ninth and dropped him to level par for the tournament, by which time Hatton was ahead again after a superb tee shot to two feet at the 195-yard par three seventh.
The Englishman moved two clear of the field with a second successive birdie but after finding the water with his drive at the 11th he careered through the green and made a double bogey to slip back into a tie at the top with Sungjae Im.
Remarkably Im, winner of last week's Honda Classic, then made a double bogey of his own after finding water with his approach at the 13th and world number 32 Hatton managed to par his way in for a memorable triumph, holing from two feet for the title.
England's Matthew Fitzpatrick produced the only sub-70 round of the weekend - with four birdies in the first six holes - and despite a double bogey at the 17th he returned a 69 to finish with a one over total and a share of ninth.