Seamus Power was three shots off the pace after carding a four-under 66 on the opening day of the Travelers Championship.
Satoshi Kodaira of Japan and Kramer Hickok each scored a 63 at TPC River Highlands in Connecticut to be seven-under and one stroke ahead of Hickok's fellow American Talor Gooch.
The Waterford native Power hit 15 of 18 greens in regulation and mixed six birdies with a pair of bogeys for his 66.
The 34-year-old made a flying start, carding three birdies in the opening four holes. A wayward tee shot on the 223-yard par-3 fifth led to his first bogey of the day, but he immediately recovered with a superb chip to within six foot resulting in a birdie at the sixth.
Following a birdie and bogey at the turn, Power should have picked up another shot at the 13th, but his five-foot putt lipped out and he had to settle for par.
His final birdie of the round came at 296-yard par-4 15th and closed with three pars.
Joint-leader Hickok, the 331st-ranked player in the world entering the week, said he had his "worst warmup session of the year" Thursday morning, prompting him to play more conservatively when he went inside the ropes. He opened with a bogey before draining eight birdies while hitting 16 greens in regulation.
"I just said, you know what? I got my C game, but just by recognizing that I had my C game I was able to play to an A level today," Hickok said. "And certainly 7-under is my A game.
"Not saying that I had my C game today, but on the range it certainly felt like it."