SHEBOYGAN, Wis. — Heading to the first tee on a bright Sunday at Whistling Straits for the final round of the PGA Championship, Jason Day was hauling a lot of major scar tissue.

He left hauling the hefty Wanamaker Trophy.

In a historical display of power and composure, Day rid himself of major championship demons by holding off Jordan Spieth and an assortment of others to win the last major of the season. Never once becoming unsettled despite the pressure applied by his colleagues, and with the thermometer reaching 90 degrees along Lake Michigan, Day closed with a 5-under-par 67 to finish at 20 under 268 and toppled Spieth by three shots.

Day’s resounding triumph carried historical weight, too. With rounds of 68-67-66-67, he became the first player to finish a major championship at 20 under, breaking the record in relation to par set by Tiger Woods in the 2000 British Open when he finished 19 under.

As Day tapped in his final shot on the brutal 18th hole, tears started to flow as his pregnant wife, Ellie, and son, Dash, ran out to congratulate him.

“The experiences that I’ve had in the past with previous major finishes has definitely helped me prepare myself for a moment like this,” said Day, who moved to No. 3 in the world rankings with the win. “I was one of those guys where I was so close and I wasn’t the last man standing and it was frustrating for me. … But for me to really kind of be patient with myself and be disciplined and give myself the opportunities, just really it does wonders for your confidence. And I’m hoping this is kind of a springboard for me to really do some fantastic, great things in the future.”

Day wanted all to forget his past failures on the game’s biggest stages — three runner-up finishes and six top-fours in his last 18 starts in majors. In the past, he was a stressful sort who forced his hand coming down the stretch, taking perilous risks instead of freely letting the outcome play itself out.

The Aussie’s frustration reached its apex at last month’s British Open on the Old Course in St. Andrews, where he didn’t make a birdie in his last 12 holes and fell one shot short of a playoff. His last bid on the 72nd hole was two inches short, leaving him dejected on the long flight home over the pond.

While doubts started to bubble to the surface about his ability to handle the heat down the stretch in a major championship’s pressure cooker, Day went north of the border and found his elixir the following week. He finished with a turkey of birdies to beat Bubba Watson by one shot to win the RBC Canadian Open. That finish was exactly the major infusion of confidence the 27-year-old Day was looking for to set him up for the last major of the season.

Playing without tension all week and having a blast on a Whistling Straits course he fancies and overpowered, Day didn’t buckle in the final round. In a nod to the thunder he started developing Down Under, Day crushed the four par-5s at Whistling Straits, finishing 14 under on them for the week. And his short game was there to save him.

Spieth, who won the first two majors of the season and was trying to join Ben Hogan and Tiger Woods as the only players to win three majors in a season in the modern era, did all he could to catch Day and finished at 17 under with a 68. Spieth joined Jack Nicklaus, Tiger Woods and Rickie Fowler as the only players to finish in the top-5 in all four majors in a single season in the modern era. Spieth also left with a very nice consolation prize — he overtook four-time major winner and defending champion Rory McIlroy as the world’s No. 1 player. McIlroy, who returned this week after he ruptured a tendon in his left ankle on July 4, shot a final-round 69 and wound up 17th at 9 under.

“It’s by far the best consolation, by far the best loss I think I’ve ever had,” Spieth said. “I played solid golf. I played 11 under on the weekend off of the tough draw the first two days and still had a chance to really win. Although the key holes were 8 through 12 for me today where I really needed to make a statement and couldn’t get it to go, I still provided some opportunities to maybe put pressure on at the end and he just shut the door.

“He was sitting there swinging as hard as he could off the tee, and every single drive was right down the middle of the fairway. I think he missed two shots today and he made two bogeys off of that. But the ones he capitalized on were good enough. … He’s impressive to watch strike the ball, but it was nothing like today. He took it back and he wailed on it and it was a stripe show. It was really a clinic to watch.”

Branden Grace closed with a 69 to finish in third at 15 under. Justin Rose, who won his first major at the 2013 U.S. Open, shot 70 to finish at 14 under in fourth, his third top-10 in a major this year. Anirban Lahiri (68) and Brooks Koepka (66) tied for fifth at 13 under.

As the heat started to tick up and the sweat start to roll, Day looked cool, calm and collected from the get-go. Starting the day with a 2-stroke lead, Day never became unnerved as the Boy Wonder from Texas never went away.

A younger Day might have caved, but even when he bogeyed the eighth and then chunked his approach on the ninth from the middle of the fairway, he wasn’t knocked off his axis. Instead Day gathered himself and made a nifty up-and-down from in front of the ninth hole and then came inward with three birdies to offset a lone bogey on the back nine.

“That was just a terrible shot that happened at the wrong time,” Day said of his chunk shot on the ninth hole. “And being able to get up-and-down definitely helped to keep the momentum going.

“ … There were plenty of times when I got out of it, more so just thinking about the future, especially on the back side where there were a few times where I had to pull myself back in and say it’s not over, you’ve got to keep grinding, keep fighting. And once I did that I kind of pulled myself back and started hitting the quality shots that I needed to. Just to be able to finish with a birdie on 16 and two pars on 17 and 18 felt fantastic.”

Day is a likeable mate on the PGA Tour and players were genuinely happy to see him win his first major. Many think it’s the first of many.

“I think obviously there’s frustration and doubt when it might happen, but I think he always knew it was going to happen,” Rose said. “He’s been very close. He had time on his side. There was no time to panic just yet.”

There is plenty of time for Day, however, to build on this win.

“I know exactly what I have done to get myself in a position where I’m holding the trophy right now. So as long as I keep working on those things and get the process right, I know that there’s going to be plenty of these to hold as long as I really am feeling motivated and I want it more than anyone else,” he said. “That’s kind of where you get that free will to go out there and just let everything fly out there on the golf course. And that’s from all the hard work that I put in before tournaments such as this.

“As long as I am healthy, I feel like I’m going to be there a long time. … It’s going to be a lot of fun over the next five to 10 years.”

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