The 25-year-old has played in seven Majors in his career, with four top-tens including a second-placed finish at the 2021 Masters Tournament.
But he had never before gone as low as 66, a feat he achieved with six birdies and two bogeys on day one at Southern Hills Country Club.
"It's super fun whenever you have days like that," he said. "I think I either made four or five 25-footers.
"It was kind of a bizarre day. I didn't drive it great early and then drove it nicely at the end but I think all six of my birdies came from the rough today, which is just very bizarre.
This is what I've wanted to achieve basically since I was a little kid, and it's kind of freeing in a way
"I'm very pleased with today, obviously. Anytime you can put up a 66 in a Major, you're obviously happy.
"They're just hard. You've got to ball-strike your way around them no matter which Major it is. Even at Augusta where the greens are incredibly undulating, you can put yourself in some pretty good spots and that's where you're going to get your best looks.
"I thought being the ball-striker I am, obviously it's shown in the first six or so Majors of my career but, on top of that, I think it's the attitude of this is what I've wanted to achieve basically since I was a little kid, and it's kind of freeing in a way.
"Obviously it's Thursday. My mind is nowhere near going to be meandering towards that but that's kind of my general attitude - you get four of these a year, they're special and I don't want to leave anything to chance."