Tyler Duncan smoothed a 6-under-par 31-34=65 during the opening round of the AT&T Byron Nelson, earning him a spot inside the top 5, just four strokes off the pace set by Marc Leishman.
And three-time PGA Tour victor Leishman settled into the fresh surroundings, not dropping a shot as he holed six birdies and two eagles.
Leishman had previously posted four scores of 62, including his opening round in last year's BMW Championship which he went on to win by five shots.
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.
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 flawless putt.
"I didn't think about that", he said.
After hitting his tee shot into the fairway bunker, Merrick hit his next to the right side of the fairway and reached the green on his third shot, rolling a two-putt bogey on the 420-yard par-4 sixth. The fairways are generous, but you still have to hit them.
"My iron play was great and then didn't feel like I was having to hit too many chip shots".
American duo J.J. Spaun and Jimmy Walker closed out the day as Leishman's nearest challengers at seven under.
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.
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.
But two-time champion Sergio Garcia needs to go low on Friday to make it to the weekend after the Spaniard mixed two birdies with three bogeys in a disappointing 72.