World number 406 Joaquin Niemann of Chile and Abraham Ancer of Mexico, ranked 196th, shared the lead after day one at the Memorial Tournament in Ohio.
Ancer and 19-year-old Niemann, making only his fifth professional start, shot seven under par rounds of 65.
They were joined by Hideki Matsuyama, who picked up six shots in five holes.
Englishman Justin Rose, the world number three, is one under, with Northern Ireland’s Rory McIlroy two over after a double bogey at the 14th.
World number eight Jason Day is the highest ranked player on the leaderboard, the Australian posting four birdies in the first six holes in a 68.
Ancer joined the PGA Tour in 2016 and has two top-10 finishes this season, including a career-best share of eighth place at the Houston Open.
Niemann, the former number one ranked amateur player, turned professional after missing the cut at the Masters in April and was sixth in his professional debut at the Texas Open, before tying for eighth last week at Colonial.
The world number 406 made all 16 of his putts from 10 feet and within at Muirfield Village and is 18 under for his last nine PGA Tour rounds.
Japan’s Matsuyama, the world number 10, had birdied four holes in a row from the 13th and then holed his second shot from 137 yards to eagle the 478-yard par four 17th.
Rose, winner of last week’s Fort Worth Invitational, started from the 10th hole alongside Tiger Woods and had three birdies in four holes before three-putting the last to drop a shot.
“I wasn’t as prepared for this week as I’d have liked but it was a solid round,” said the 37-year-old former US Open champion.
Woods, a five-time winner of the event, sent his tee shot out of bounds at the par five 15th – his sixth – en route to a double bogey.
However, the former world number one, currently ranked 83, had three consecutive birdies on his back nine and finished level.
“I wasn’t rotating well, my back was tight,” the 42-year-old explained. “I have days like that, my back is fused. I don’t have any pain, thank God, it’s just tight.”
World number one Justin Thomas also carded a double bogey in a level par round.
But his close friend Jordan Spieth, the three-time major champion, was three over, despite a 57-foot birdie putt at his opening hole.
McIlroy, the world number six, was one under before he put his tee shot with an iron into the creek for a double bogey six and the 29-year-old saw another shot go at the last.
Crowd favourite Phil Mickelson was four under after eight holes but also ran up a double bogey and then dropped four shots in five holes on the back nine to join McIlroy on two over.