That’s the lowest score any player has carded on the first day of the six previous US Opens at Winged Foot, with the world No. 3 finishing one shot clear of Patrick Reed, Thomas Pieters and Matthew Wolff and two clear of Rory McIlroy and Lee Westwood.
Thomas, 27, is looking to add to his sole major triumph at the 2017 PGA Championship.
“65 is fun no matter where you play, especially at Winged Foot,” he told reporters.
“I was in a really good frame of mind, and I was focused. I just was sticking to my routine and playing every shot, as opposed to getting ahead of myself.
“It’s one of those rounds where it’s just kind of like, next thing you know, you make the putt on 18, you’re done for the day.”
Good conditions meant players enjoyed lower scores than many had anticipated at Winged Foot, a course that has seen three over-par winning totals at the US Open in the post-war era.
There were holes-in-one on Thursday from Reed and fellow American Will Zalatoris on the seventh hole — the 46th and 47th aces in the competition’s history.
Reed landed his after one hefty bounce on the green, while Zalatoris saw his roll in on the way to an evens finish.
Former Masters champion Reed was one of 21 players to go under par. Close behind him was 2011 US Open winner McIlroy, who is now without a major title for six years.
“First round of a major you’re always anxious to play well, and maybe I’ve overthought it at times,” the Northern Irishman said of his good start.
“I just went out today and just took what was given to me a little more relaxed and played really nicely.
“I think at a US Open, if you can get off to a good start, you’re not chasing as much. And when you chase on US Open golf courses, that’s when you can start to make mistakes and compound your errors.
“To have that sort of cushion, to be a little bit more relaxed about your play, not take on too much, be able to still play conservative golf, I think that’s important here.”
Tiger Woods, who has struggled for form since golf resumed earlier this year, was disappointed to conclude his round on three-over after ending on a bogey and double bogey.
“I needed to finish off the round better, and I didn’t,” he told reporters.
“As I said, I made a few putts the middle part of the round. Seemed like I wasn’t getting anything out of my round early on, and it flipped, and unfortunately I just didn’t finish off the way I needed to.”
Meanwhile, Phil Mickelson’s strained relationship with Winged Foot — where he let slip a US Open lead on the final hole in 2006 — continues. He shot a nine-over 79.