Leishman heads Venezuelan Jhonattan Vegas and American Brian Harman by one stroke, with world number one Dustin Johnson among a trio two behind.
While Johnson started solidly, some of the big names in the 34-man field battled to dust off Christmas rust, none more so than Jordan Spieth.
World number two Spieth recorded his first ever over-par score at Kapalua, a 75 that left him ahead of only three players in the small but elite 34-man field.
Spieth said he had made some "significant changes" in his putting set-up over the past month that he expects will take several rounds to feel comfortable.
While Spieth struggled, Leishman offered a reminder that behind the superstars lurk a bunch of players capable of winning any given week.
"Played well, drove ball pretty well with exception of a couple of drives and putter was really solid," Leishman told Golf Channel. "Happy with eight birdies, good start to a nice tournament."
He starts 2018 the highest-ranked Australian, 12th in the world, after a hot finish to 2017 that included a victory, a second, a third and a fourth-placing in his final six global starts.
Leishman won the BMW Championship in September, and his runner-up finish was a playoff loss to Thomas at the CJ Cup @ Nine Bridges in South Korea in October.
"Really high," he said of his confidence level. "Good results help with that obviously. I know I'm not owed anything but confidence certainly make it easier to play well."
Vegas is also coming off a good year during which he climbed into the top 50 in the world, with he hopes more to come.
Like Leishman, he knows he can beat the best when he hits it straight.
"It's a big key for my game to drive it well," Vegas said.
"I hit my irons as well as some of the best on tour and if I'm able to keep it in play, and give myself good positions to attack hole locations I'm going to get a lot of birdie opportunities.
"Last year was pretty good year for me, moved into top 50 in the world. This year I want to move even higher, somewhere in the 20s."
(Reporting by Andrew Both in Cary, North Carolina; editing by Amlan Chakraborty)
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