Argentina's Emiliano Grillo and England's Matt Fitzpatrick are the joint-leaders after the first round of the Arnold Palmer Invitational in Florida.
Grillo, the 2016 PGA Tour Rookie of the Year, birdied five of the first 11 holes and two of the last three en route to his five-under 67.
Fitzpatrick went through bogey-free in the opening round of the event, the first since Palmer's death in September.
Paul Casey and American duo Lucas Glover and Charley Hoffman are tied for third, one shot back.
Florida-born Australian Ryan Ruffels, 18, and compatriot Greg Chalmers are at three under.
In unusually cool conditions at Bay Hill, defending champion Jason Day posted a two-under 70.
Making a much poorer start was Rory McIlroy, the four-time major champion producing a rollercoaster back nine to card a two-over 74.
The Northern Irishman was one under through nine, but he bogeyed the 10th before finding a bunker and water at the next for a double bogey.
He had birdies at 13 and 16, but bogeys at 15 and 18 saw him end up in a tie for 58th.
Cody Gribble, who had a forgettable five-over 77, produced one of the moments of the day, nudging an alligator back into the water.
Sharing 58th place with McIlroy is Palmer's grandson, Sam Saunders.
Reigning Masters champion Danny Willett, meanwhile, was a late withdrawal due to illness.