Former two-time PGA champ Thomas had looked set at one point on Thursday to be the biggest victim of the damp, difficult conditions as no-one was able to launch a charge, yet he rallied strongly to take an early one-shot lead with his three-under par 68.
The 31-year-old had made the best start of any of the big names, powering to four under just after the turn, but he couldn't sustain it, running into trouble with a double-bogey six at the 12th and a bogey at the next.
Yet a tee shot to eight foot at the 17th and a 25 foot putt for birdie at the last inched him into the lead, ahead of Sweden's Alex Noren, Denmark's Nicolai Hojgaard and Britain's former US Open champ Justin Rose, who all recorded 69s.
Australia's evergreen veteran Adam Scott also made a good start in his 24th successive Open at the age of 44, beginning with a splash-in from the greenside bunker at the first for a birdie, the launchpad for him going round in one-under 70.
What a start.— The Open (@TheOpen) Adam Scott holes out for a birdie on the 1st.https://t.co/ngME6k3h28 pic.twitter.com/3eAdstHIKSJuly 18, 2024
Three birdies in the final four holes also helped another Australian Min Woo Lee battle back to level par in his fighting round.
But Bryson DeChambeau, fresh from his triumph in last month's US Open, looked to be in big trouble after reaching halfway at six over par, having dropped five shots in his first six holes.
Like Thomas, South Korean Song Young-han did also get to four under but he was derailed by a triple-bogey seven on the tricky par-four 11th, while England's Matt Wallace, who enjoyed a chip-in eagle at the fourth, also lost three shots in one hole at the ninth.
Reigning champion Brian Harman, whose exceptional putting was largely responsible for him winning at Royal Liverpool, shocked everyone by missing a short putt on the first and he was one over after eight.
World No.1 Scottie Scheffler was among the late starters, along with Australia's top hope Cameron Smith, the 2022 champion.
American Justin Leonard struck the opening tee shot, but the 1997 winner smiled that it was his only highlight as he finished with a nine-over 80.
