The Australians are three behind leaders Jordan Spieth and J.J. Spaun, on a day when only 12 players went over par in the 156-man field.

World No.38 Leishman began his round with a birdie and then finished strong to lurk ominously.

The Victorian was particularly brutal on his closing nine when he eagled the par-five fifth and followed it up with birdies on the sixth, eighth and ninth.

Gibson also began on the back nine and birdied the 12th, 14th and 16th before continuing his birdie push on the third, fourth, sixth and ninth.

He's lone blemish was on the par-four eighth where he made bogey, which saw him finish tied 19th.

RIGHT: Having clearly recovered from his recent bout of COVID-19, Spieth shares the lead with compatriot J.J. Spaun after round one. PHOTO: Matthew Stockman/Getty Images.

Former World No.1 Spieth hasn't played much golf lately with the three-time major winner revealing this week he had tested positive for COVID-19 about 10 days after the Masters.

Despite him taking a few weeks off to recover, the virus neither sapped his golf talent nor his flair for the dramatic.

Spieth drained a 55-foot eagle putt on the final hole to cap off a bogey-free, 63 that vaulted him into a tie with Spaun.

"I had nerves on No.1 like it was the first shot of the season," Spieth said. "I didn't practise for three weeks straight, which is very unusual. Normally two days off is the most during a season."

Spaun also finished without a bogey, and he holds at least part of the lead at a PGA Tour event for the third time in his career. He has yet to earn a win.

Spaun has missed nine of 16 cuts this season but came to Dallas off an encouraging showing at last week's Wells Fargo Championship, where he tied for 18th.

The 30-year-old said he'd been "spinning (his) wheels" trying to fix his game, but soon stopped himself from overthinking things.

"It was a lot of searching," Spaun said.

"And I feel like that's something that us golfers kind of do too much of when things start to go wrong, instead of just chalking it up to a bad week or a just kind of a couple-week slump with the striking."

Spain's Rafa Cabrera Bello and Americans Doc Redman, Aaron Wise and Joseph Bramlett are in a tie for third after shooting 64.

A group of 12 golfers – two-time Byron Nelson champ Sergio Garcia of Spain and former World No. 1 Luke Donald of England among them – are at seven under.