Thomas is the defending champion, courtesy of an 8-iron from 214 yards that settled three feet away for birdie on the 17th hole that secured his three-shot victory and sent him on his way to a season that ended with his first major (PGA Championship) and being voted PGA Tour Player of the Year.
Johnson slipped back to two under for the tournament when he bogeyed the third and fourth, but the world No 1 got back on track with three birdies in four holes before picking up four more on the back nine, including a sublime pitch to within a foot at the last from close to 100 feet.
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.
Jhonattan Vegas and Pat Perez are a shot further back, while Rickie Fowler lost ground on the leaders as he slipped to six-under after a two-under 71.
"Played well, drove ball pretty well with exception of a couple of drives and putter was really solid", Leishman told Golf Channel.
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"So, happy with eight birdies", added Leishman after a day that saw strong winds buffet some of the bigger names in the field.
British Open champion Jordan Spieth struggled to a two-over par 75.
Fowler joined in the frat house-style revelry that followed Spieth's wins in the Masters, US Open and Open, plus that of Thomas in the US PGA.
Global challengers include Japan's Hideki Matsuyama, Spain's Jon Rahm, Australians Marc Leishman and Cameron Smith, Sweden's Jonas Blixt and South Korean Kim Si Woo.
The 34-year-old ended the second round on 10-under for the PGA Tour event but it could have been so much better had he not missed an eagle opportunity at the 18th. "Hopefully I can keep this going". "I know I'm not owed anything but confidence certainly make it easier to play well".