Coming off his best year, Marc Leishman got off to an ideal start in 2018 with eight birdies and a 6-under 67 for a one-shot lead in the Sentry Tournament of Champions.
World number one Dustin Johnson lies in second place at nine under, despite back-to-back bogeys on the third and fourth.
Leishman strung together four birdies in a row from the seventh through the 10th holes on the par-73 Plantation Course at Kapalua on Maui.
Defending event champion and 2017 PGA Championship victor Thomas, US Open champion Koepka, Spieth and long-hitting Dustin Johnson lead a set of US stars into the 34-player field at Kapalua for the first US event of 2018.
"I played well, drove the ball pretty well - with the exception of a couple of drives - and the putter was really, really solid".
He said he was happy for the wind to blow at Kapalua, as the forecast suggests it will over the next three days. He finished the day with a 22-foot eagle putt on the 18th to get into the clubhouse at four-under. 11 and 13. He missed a four-foot putt to bogey the 11th hole and then hit a series of ugly approaches on his way to a double on No. 13.
British Open champion Jordan Spieth struggled to a two-over par 75.
Johnson is chasing first-round leader Marc Leishman, as well as Brian Harman, who are tied at the top after 36 holes at 10-under 136.
The 34-year-old Australian enjoyed his best PGA Tour season of his career last season, winning the Arnold Palmer Invitational and the BMW Championship.
Defending champion and world No.3 Justin Thomas limped to a 75 and falls to even-par overall.
While Leishman said his recent strong results didn't mean he was "owed anything" in 2018, he said he was looking forward to the year with confidence.
"I think the hardest part is going to be staying in the moment and recognizing that it's a new year", Thomas said as last year was ending.