Scott Piercy birdied seven off his opening 11 holes on his way to hold a share of the lead at the WGC-Cadillac Championship, where Rory McIlroy sits five strokes off the pace.
Piercy's fast start at Doral's 'Blue Monster' course helped him to a six-under 66 and set the clubhouse target, only for Marcus Fraser to move alongside him late in the day.
After following four gains on his opening five holes with a seven-footer at the eighth, Piercy kicked off his back-nine with successive birdies to temporarily go three clear of the rest of the field.
The American bounced back from his dropped first shot of the day after a wayward drive at the 14th to birdie the 17th, only to card a second bogey at the last when driving his tee shot out of bounds.
Fraser looked set to claim the outright advantage when a hat-trick of birdies from the 14th lifted him to seven under, only for the Australian to find the water and bogey the par-five eighth before two-putting his final hole for par.
Phil Mickelson, who has not won since claiming his fifth major title in the 2013 Open at Muirfield, sits a stroke behind in third, with Danny Willett and former Masters champion Adam Scott in the group on four under.
"I hit a lot of good shots, I was really pleased with the way the round went," Mickelson told Sky Sports. "I probably let a few shots go early on, but I hit a lot of good shots.
"I was patient with the round and on my second nine I was able to make some birdies and capitalise on some of the shots."
Jordan Spieth was the pick of the marquee group alongside McIlroy and Jason Day, mixing five birdies with to bogeys on his way to a three-under 69, as McIlroy stuttered along the closing stretch to post a one-under 71.
McIlroy jokingly threatened to throw his putter in the water after three-putting the eighth, a reference to last year's incident when his three iron was despatched in to the hazard, before the world No 3 sent his tee shot at the next in to a watery grave to double-bogey his final hole.
Back-to-back bogeys after the turn saw Day card a level-par 72, leaving the Australian in a tie for 28th that includes defending champion Dustin Johnson and Billy Horschel.
Watch the WGC-Cadillac Championship live throughout the week on Sky Sports 4 - your home of golf. Live coverage gets underway on Friday from 6pm. Or watch from £6.99 without a contract, on NOW TV.