World number three Jason Day carded a 66 to earn a one-stroke lead after the opening round of the Arnold Palmer Invitational on Thursday.
Day shot six under to hit the front ahead of fellow Australians Marc Leishman and Adam Scott at the Bay Hill Club, as Rory McIlroy struggled.
Back in action after finishing tied for 23rd at the WGC-Cadillac Championship a fortnight ago, Day recorded seven birdies and an eagle in Orlando.
The 28-year-old did double-bogey the par-four ninth, having dropped a shot at the second, but the 2015 US PGA Championship winner responded by shooting a five-under 31 on the back nine.
Countryman Leishman and in-form Scott are in striking distance of Day, alongside Brendan Steele, Henrik Stenson and Troy Merritt.
Leishman and Scott - who is bidding to win a third consecutive PGA Tour title - were bogey-free in the opening round, as well as Swede Stenson.
Justin Rose headlines a group of 10 players two shots off the pace, which includes Francesco Molinari, Chris Kirk and Kevin Chappell.
Matt Every's quest for a third consecutive triumph at the event started with a first-round 71.
Every had a mixed bag with three birdies and two bogeys leaving him five shots behind Day.
Meanwhile, world number two McIlroy will tee off on Friday three over following his 75.
McIlroy - who has played five events this year for two third-placed finishes - never got going, having double-bogeyed the opening hole, which he repeated at the eighth.