Tony Finau set the first-round pace at the Memorial Tournament on Thursday, shooting a six-under 66 that left him four ahead of Rory McIlroy, with Tiger Woods a shot further back.
Finau picked up six birdies at Muirfield to sign off a shot better off than fellow American Ryan Palmer.
McIlroy is part of a cluster of ten players who are two-under after opening with solid 70s.
Shane Lowry meanwhile bogeyed two of his last three holes to get home with a one-over 73. Graeme McDowell, who endured a very tough afternoon, is seven-over-par.
On a day that saw tough scoring conditions due to gusting winds at Muirfield Village, 30-year-old Finau stepped up his play on the greens.
"I made a lot of putts. I honestly didn't feel I hit it great off the tee. I didn't hit a lot of fairways," said the American, who mixed nine birdies with three bogeys.
"But I felt great out there. It was playing really tough but ... I was able to execute pretty well."
Finau is one clear of Ryan Palmer, who had a bogey-free round of 67. Early leader Brendan Steele and 2019 U.S. Open champion Gary Woodland lurk two back after rounds of four-under 68.
Tiger Woods, back in action for the first time since February's Genesis Open, came out blazing with birdies on two of his first three holes but mostly failed to take advantage of the course's par-fives.
"It's been a while since I've played," Woods said.
"Got off to almost an ideal start and got a feel for the round early. I just didn't make anything today. I had looks at birdies, but I really didn't make much."
His round ended on a high note when he coolly sank a 14-foot putt to finish in red numbers and tied for 18th.
Woods is gunning for a record-breaking 83rd PGA Tour win this week.
Former champion Bryson DeChambeau, one of the favourites coming into the Jack Nicklaus-hosted event, was one over after a round which included a monstrous 423-yard drive that led to a birdie at the par-four first hole.
Collin Morikawa, winner of last week's Workday Charity Open, struggled on Thursday, carding a four-over 76, his highest round as a professional on the PGA Tour.
