World No. 16 Leishman, one of only four top-20 players competing this week at the links-style Trinity Forest Golf Club, carded a bogey-free 10-under-par 61 to sit three shots clear of Americans J.J. Spaun and Jimmy Walker. "I didn't want to let myself get ahead of myself which was good".
The three-times PGA Tour victor, whose previous career-low was 62, then made birdies at the sixth and seventh holes before a scorching trip around the back nine that included four birdies and an eagle at the par-five 14th.
Jonathan Randolph hit 14 of 18 greens in regulation during his first round at the AT&T Byron Nelson, finishing at 1 over for the tournament.
"It was about as easy tee to green as I've ever seen this place", Spieth said.
And now he leads this year after an opening 61, which contained eight birdies and two eagles, saw him card the best round of the day by a good distance.
Leishman, whose previous TOUR low was 62, acknowledged it was one of his best rounds but couldn't match the 64 he shot in the third round of the 2015 Open Championship.
The Byron Nelson is being staged at its new home - Trinity Forest - for the first time after bidding farewell to the Four Seasons Resort and Club. "The fairways are generous but if you are off them you are in trouble".
Former U.S. Masters champion Adam Scott, seeking his first PGA Tour win since 2016, mixed six birdies with two bogeys to reach four-under and a share of 23rd.
He hit his second shot on the par-5 14th to 13 feet and drained the putt for eagle before sinking a birdie putt on the par-3 17th.
Marc Leishman has a penchant for low scores, but the Australian charted new territory today.
"I was hitting my irons well and hitting every fairway is key around here".
Leishman was in a tie for 63rd at last week's Players Championship and he was in contention after two rounds of last month's Masters Tournament before finishing ninth, seven shots behind the victor Patrick Reed.
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