There were two more bogeys on the back nine.
The Englishman may have shot a one-over par 72 on Sunday for an eight-under total - it's the nature of the Copperhead Course in these fiery conditions - but it was enough to win by one shot from American Jason Kokrak and South Africa's Louis Oosthuizen, who remains in search of his maiden victory on U.S. soil, a remarkable statistic for the former Open champion who has regularly contended in majors across the Atlantic. Who can argue? Casey, who said he's getting older but better, picked up his third career PGA TOUR title in his 250th start.
The 41-year-old Englishman stumbled with a bogey at the par-3 17th hole but two-putted from 22 feet at 18 to become the first back-to-back victor of the event at Innisbrook Resort's Copperhead course in Palm Harbour, Florida. And that was after he eagled the first hole.
Johnson was never a threat, uncharacteristically failing to post a single birdie, in a three-over par 74 to end in a share of sixth place at five-under.
It was Oosthuizen who would then make the first mistake, missing a putt from 8 feet for par on 16 to drop back to seven-under as Casey remained one shot ahead of Kokrak. Casey's start didn't go as planned.
The Englishman was level with Jason Kokrak at eight under on the 18th hole but the American made bogey, meaning Casey needed par to win. He used an eagle from the green-side bunker on the par-5 1st hole to start his round and keep the momentum despite two bogeys. He dropped 25 spots to a tie for 46th. She won when third-round leader Yu Liu missed a 15-foot par putt on the par-4 18th.
"The bunker shot on 18 was pretty damn good", Casey said. He chose to play conservatively, clearing the lip of the bunker and hitting his approach to 22 feet.
The two victories, although separated by just 12 months, couldn't be more different. Outdueling Tiger Woods on that day last March felt for Casey like a new beginning to his career.
Obviously, I'm disappointed with the outcome but had I known this time yesterday that a round of 70 would have won him the title I'd have had considerably more on than I did!
"Today wasn't easy", Casey told NBC Sports.
"This feels very different from previous year, it feels better, I'm enjoying this one so much more", Casey said after his round. "Mentally a year ago I was hoping I would win, wanting to win".