The 32-year-old Englishman was the only player who had any hope at catching Louis Oosthuizen in the final round at the Old Course. With no way to hit out of the thick bushes left of the 12th fairway, a penalty drop was his only option. Unnerved, Casey needed five more to finish off a triple-bogey 7.
Oosthuizen made birdie at the same hole, and — just like that — a four-shot margin grew to a staggering eight. The engraver had a little extra time to put a name on the claret jug that few people knew before this week.
Even if that shot hadn't veered off course, it's unlikely that he, or anyone else, would have caught Oosthuizen. The barely known South African put it all together for one magical week, and Casey would have needed something akin to a 67 — as good as anyone shot on Sunday — to have any chance of wiping out the deficit he faced at the start of the round.
"As disappointed as I am with the way I played today, Louis was in a different league," said Casey, who struggled home with a 3-over 75 that left him tied for third. "You know, that softens my disappointment slightly, because it was a tremendous performance. Hats off to him."
Casey's pain was eased even more by the state of his game.
He was No. 3 in the world heading into last year's Open, but strained a rib muscle the week before. Casey made it through four painful rounds but could only manage a tie for 47th at Turnberry. After rest and treatment, he tore the muscle at Firestone and had to withdraw. He withdrew again before the PGA Championship.
Casey didn't play another full round until the World Match Play Championship in October, where he lost all three of his rounds.
"I'll be honest, it was scary," he said. "I was very worried about it. I thought, 'Is this something I'm going to be battling the rest of my career? Am I done? Will I ever be pain free?' I had no idea. I was pretty down."
Casey has rebounded this year, with five top-10 finishes around the world, but his performance at the birthplace of golf makes it clear he's all the way back.
Now, all he has to do is win that first major title.
"Six months ago, I had no idea if I was going to get back to this sort of form again," he said. "Now I know I'm going to win a major. It's just a matter of time. This week just wasn't my week."
Casey's chances were largely undone by two of the 72 holes. He took a triple-bogey at No. 17, the famed "Road Hole," in Friday's second round. Then came No. 12 on Sunday.
It was bad enough that he drove one straight into the gorse. Casey was actually more perturbed about what he did after that. Taking a drop behind the bush, he came up short on his approach to the green. Then he chipped it past the green. He left his putt 4 feet from the hole, but missed that one, too. In a matter of minutes, he had another triple on his card.
"I should have been making 5, at worst 6," he said. "That was very annoying."
After watching Casey knock it all over the place, Oosthuizen knew victory was his. To eliminate any doubt, he rolled in an 18-footer for birdie.
"All of a sudden, it was mine to throw away," the winner said. "It was actually very tight until the 12th hole. I mean, it could easily be: I make bogey and Paul makes birdie, and it's a (two)-shot game. The minute he made that number, that putt to me meant a lot, that birdie putt, just for momentum on the next six holes."
Casey kept grinding, hoping the stiff wind sweeping across the course might throw Oosthuizen off his game. But when Casey sent what he thought was a perfect 6-iron soaring toward the 16th green, then watched it roll back into the valley, he knew it was over.
"I was like, OK, clearly I've got to back away from the sharp objects," he quipped.
There will be other majors, other chances to break through. Next up is the PGA Championship, where Casey will be eager to make up for missing the cut in 2004 when that major was last held at Whistling Straits.
Oosthuizen, who spent much of the round chatting amiably with Casey, has no doubt his friend and rival will have his moment.
"Paul is a fantastic golfer and a great person," Oosthuizen said. "He's definitely going to win a major, that's for sure. It's always nice playing with him. We have a lot of fun on the course, talk about other things. I think it's important. It's still just a game you're playing, and you've got to have fun with the guys you're playing. Otherwise, it's going to be quite miserable out there."