Thomas rolled in 11 birdies and tied the single-round course record with a 10-under-par 61 at the South Carolina course, narrowly missing a six-foot putt right of the hole for a record-setting 60 after dropping in a series of shots with the short stick from substantial distance. Thomas took par at 18.
He got himself within striking distance of the record with a birdie putt from 38 feet on the 17th before his miss at the last, which was still good for the lowest opening-round score in RBC Heritage history.
David Frost (second round, 1994) and Troy Merritt (second round, 2015) share the record for 18 holes in the 57-year history of the tournament.
"I just played really solid," Thomas said.
"I feel like I didn't do anything crazy. I just drove the ball well, which is very, very important out here, and I felt like it was just one of those days I put the ball in a spot that I had a lot of good numbers."
Thomas was three shots clear of Scottie Scheffler and Russell Henley, who ended the morning wave tied for second place at seven-under (64).
Thomas made the turn with a share of the lead at six-under (30), comprised of six birdies and three pars, but opened the back with bogey on No. 10 before settling back in by taking birdie on five of seven holes.
Defending champion Scheffler made the turn trailing by one shot but quickly pulled even at 6-under with a 21-foot birdie putt.
He had seven birdies and played bogey-free on Thursday. Henley, who won the Arnold Palmer Invitational last month for his first Tour win since 2022, kept pace with birdies on four of his final fiver to close out a round of 64.
It was Thomas' 14th career round of 62 or better, most on the PGA Tour in the past 40 years. Kevin Na (11) is next in line with Tiger Woods and Zach Johnson among a group with nine rounds of 62 or below.
Thomas tied the course record at TPC Sawgrass with a 62 in March that he secured with 11 birdies one day after he shot 78.
Matt Fitzpatrick, winner of the 2023 RBC Heritage, shot 66 and was part of a logjam five off the lead. England's Fitzpatrick chipped in from 65 feet for eagle on the par-five second and birdied two other par-fives in a bogey-free round.
Thomas finished tied for fifth in 2024 with a 72-hole score of 14 under and a single-round best of 6-under in the final round. Scheffler won the event at 19 under, three clear of runner-up Sahith Theegala.
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