Thomas began the day on 5-under, but had two bogeys and a birdie in the opening three holes.
"It wasn't the way I planned that 18th hole going", Day said in an interview posted on the Sydney Morning-Herald.
"I have to work on my mental game". "And 13 was really special as well".
Matsuyama, who matched the best Japanese showing in a men's major with his US Open runner-up effort two months ago at Erin Hills, fired a final-round 61 last Sunday to win a World Golf Championships title. The pins were receptive, though.
On Sunday, when I thought I will try and fight and create some momentum, I just did not get going from the start and worse I had nine bogeys. He won the CIMB when I was in contention late past year and then won the Hawaii double and now the PGA.
"I've missed a lot of time already".
Overnight leader Kevin Kisner (74) finished four shots back.
Thomas' 8-foot birdie putt on the 10th green hung on the left lip for almost 10 seconds before dropping into the hole.
On the 10, Thomas had looked on in disgust as his ball stayed on the lip.
"I want to get back into that winner's circle", the 28-year-old said.
Kevin Kisner was the last one who had a chance to catch him.
"It was a insane day". "It had to be an unbelievable day spectating". Oosthuizen, the only major victor among the contenders after his 2010 British Open triumph, holed a 34-yard chip shot eagle at the par-5 15th but followed with a bogey to blunt the impact of his closing birdie.
"(Woods) hit the putt on camera and before it can fall in on TV, I can just hear the roar outside. "I gave it everything I had coming in". Matsuyama finished one stroke better than Kisner, in a tie for fifth place, after sharing the 36-hole lead and starting the final round one stroke back.
He had earlier finished with a closing 70 for two-over-par.
The world's 14th-ranked player closed with a 3-under 68 that included four birdies in a six-hole stretch.
Thomas' play with the flat stick was the difference, as he had just 10 putts on the back nine through 17 holes.
McIlroy was expected to fly home to Northern Ireland on Sunday and is due to meet Steve McGregor, his fitness expert and sports scientist, within the next few days.
"It's pretty emotional for me to be quite honest", Alabama head coach Jay Seawell said. I was lucky enough to see it in person and it was spectacular.