Thomas stands one shot clear of South Korea's Sungjae Im and has a stroke on five players tied at six-under: Sweden's Ludvig Aberg, American Justin Lower, Germany's Maximillian Kieffer, China's Haotong Li and Belgium's Thomas Detry.
Adam Scott and Min-Woo Lee are five off the lead, tied for 34th, with Cam David and Jason Scrivener a shot further behind on two-under.
"It's always good to get off to a good start, and even better to get off to a great one," Thomas said of his bogey-free round. "I felt like I was in great control of everything. Just when I missed it, I missed it in the right spots and had a lot of really quality shots."
Aberg is tied for fourth or better for the third time in his last four starts.
His approach on eight would have landed inside of 15 feet to the cup had it not landed directly on top of Morikawa's resting ball, kicking both further from the cup in opposite directions. Instead of a birdie putt, Aberg wound up with a one-of-a-kind bogey. Morikawa made his putt for a birdie on the par-4.
"You can't really see the bottom of the flagstick from the fairway. So I didn't see it," Aberg said. "I thought I hit a decent shot, and we just saw the ball just go straight sideways. And I didn't really know what it did but then we kind of realised that's pretty much where Collin's ball was and then I saw his ball go that way. So it's just one of them -- I've never seen it before. Probably not going to see it for a long time again."
Thomas posted his best first-round score since 2017 in ideal playing conditions: 58 degrees, dry and moderate wind speeds.
"This is as easy as you're going to get a links golf course weather-wise and conditions. When you drive it well like I did for the most part today, you have a lot of short clubs, and I see nothing but the pin," Thomas said.
Defending champion Rory McIlroy of Northern Ireland, Briton Tommy Fleetwood and American Morikawa are among the 15 players tied for eighth at five-under.
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