England's Matt Fitzpatrick and American Webb Simpson took a share of the lead in a star-studded field at the WGC-Workday Championship.
World number 16 Fitzpatrick carded a six-under 66 without dropping a shot on day one at the Concession Golf Club in Bradenton, Florida.
Ninth-ranked Simpson closed a clean back nine with three straight birdies and a par to climb up the leaderboard and match the 26-year-old Englishman's score.
Four-time major winner Brooks Koepka dropped his only shot at the par-four 16th, but is alongside three others at five under.
Americans Kevin Kisner and Billy Horschel also carded 67s and were joined in third place by 2017 Masters winner Sergio Garcia, who drained a round-high eight birdies.
World number two Jon Rahm and Patrick Reed were among six players tied in seventh at four under, along with Tony Finau.
Adam Scott saw a potential hole-in-one come back off the flag at the sixth and shot an up-and-down even-par 72, while Justin Thomas recovered from dropping four shots in three holes to close with three birdies in the last four to sign for a one-over 73.
World number one Dustin Johnson posted a pair of double bogeys on his way to a five-over 77. That score was matched by US Open champion Bryson DeChambeau, who dropped shots on seven holes.
Rory McIlroy made a decent start, shooting a three-under 69 to sit in a tie for 13th.
The first World Golf Championships event of the season kicks off the Florida swing of the tour, heading towards The Players Championship starting on March 11.