Former US Open champion Webb Simpson surged late for a share of the first-round lead on Thursday in the World Golf Championships-Workday Championship, alongside England's Matthew Fitzpatrick.
Fitzpatrick had six birdies without a bogey on the par-72 Concession Golf Club course in Bradenton, Florida.
He was joined on 66 by Simpson, who had four of his six birdies on the back nine, including three in a row at the 15th, 16th and 17th.
"Very pleased with the start," said Simpson, the 2012 US Open champion whose seven PGA Tour titles include two last season.
"The course is fantastic," he said of a course that was pressed into service when coronavirus complications forced officials to relocate the scheduled WGC-Mexico.
"It's firm. I was hoping it would get firm and fast. It takes that lengthy scorecard down a little bit."
Fitzpatrick, a six-time winner on the European Tour who is seeking his first US PGA Tour title, teed off on 10 and had four birdies in his first nine holes.
His two birdies coming in included a 37-footer at the sixth.
"I thought I played really solid today," said Fitzpatrick, who was in contention at Riviera last weekend on the way to a share of fifth in the Genesis Invitational.
"I feel like I took a lot from my win at the end of the year in Dubai and I feel like my game was in really good shape there and we kind of found something that gave me a really good feeling."
He said the fast, sloping greens at Concession are a challenge, but one that he enjoyed.
"I think it makes you think, this golf course," he said. "You've got to think about where you're hitting your tee shots and your approach shots. You've got to be on the ball."
Former world number one Brooks Koepka, who won his first tour title since 2019 at the Phoenix Open this month, was a stroke back on 67 alongside former Masters champion Sergio Garcia of Spain and Americans Billy Horschel and Kevin Kisner.
World number two Jon Rahm of Spain had six birdies and two bogeys in a 68. He was joined at four-under by South Korean Im Sung-jae, Australian Cameron Smith, Tony Finau and Patrick Reed - who is defending the title he won in Mexico last year.
Some of the field's biggest names found the going tough.
World number one Dustin Johnson had two double-bogeys in his five-over 77 - where he was joined by two-time Masters winner Bubba Watson and reigning US Open champion Bryson DeChambeau - whose card featured a triple-bogey and three bogeys.
World number three Justin Thomas struggled early in the round with a double-bogey and two bogeys in a three-hole span, but birdied three of his last four holes for a one-over 73.
Leading first-round scores on Thursday in the World Golf Championships-Workday Championship at Concession Golf Club in Bradenton, Florida (USA unless noted, par-72):
66 - Webb Simpson, Matthew Fitzpatrick (ENG)
67 - Brooks Koepka, Billy Horschel, Sergio Garcia (ESP), Kevin Kisner
68 - Wade Ormsby (AUS), Im Sung-jae (KOR), Cameron Smith (AUS), Patrick Reed, Jon Rahm (ESP), Tony Finau
69 - Scottie Scheffler, Sebastian Munoz, Joaquin Niemann (CHI), Rory McIlroy (NIR), Victor Perez (FRA), Louis Oosthuizen (RSA)
70 - David Lipsky, Lanto Griffin, Collin Morikawa, Jason Kokrak, Thomas Detry (BEL), Tyrrell Hatton (ENG)