Leishman shot a 10-under-par 61 to hold a three-stroke advantage after the opening round at Trinity Forest in Dallas.
Marc Leishman has fired the best round of his PGA Tour career to storm into the lead at the Byron Nelson in Texas.
The 34-year-old Victorian, a three-time victor on the PGA Tour, started with an eagle on the par-5 1st hole and never looked back, claiming another eagle at the 14th to go with six birdies.
Marc Leishman shot his lowest round on the PGA Tour.
The Australian was a flawless 14 from 14 in fairways hit, and his opening hole set the tone for his round as he hit a superb 236-yard second to three feet and rolled in the putt for eagle.
Leishman's iron play was in good order as he set up three birdies in a row from the 10th, and he added a second eagle at the 14th following another delightful long-iron to 12 feet and a ideal putt.
American duo J.J. Spaun and Jimmy Walker closed out the day as Leishman's nearest challengers at seven under.
"It was certainly one of the best driving days I've had", said Leishman. The fairways are generous, but you still have to hit them.
"To play like I did today, I didn't hit it in too many bad spots so I think that makes it a lot easier", Leishman said.
John Merrick hit 13 of 18 greens in regulation during his first round at the AT&T Byron Nelson, finishing at 1 over for the tournament.
"I was hitting the right side of the fairways (with) good iron play and I obviously had a hot putter as well".
Defending champion Billy Horschel, who started on the back nine, had an eagle at the par-five 14th en route to a three-under 68 that left him seven shots back.
A three-time major victor, Spieth - who was tied for 41st at The Players Championship - had three birdies and a bogey on a steady day, when Leishman sizzled.
As for two-time champion Sergio Garcia, the 2004 and 2016 victor recorded a 72 to be 11 shots adrift.