Matt Every shot an opening-day 65 to claim the overnight lead in the Arnold Palmer Invitational at Bay Hill in Orlando.
The American, who started on the back nine, had picked up three shots by the turn and birdied three of the next four holes to join clubhouse leader Rory McIlroy on six under par before edging his way in front with a three at the par-four eighth.
McIlroy, who also played the back nine first, dropped a shot at his second hole but was one-under at the turn and and collected an eagle three at the fourth as he collected five more to finish with a six-under-par 66.
Americans Talor Gooch and Scottie Scheffler will head into the second round tied for third, a shot further back, with Englishman Tyrrell Hatton and Northern Ireland’s Graeme McDowell among a group of players on four under and Ian Poulter a shot worse off.
However, Justin Rose may need an improvement to make the cut after shooting 73, while Padraig Harrington, Jason Day and Lee Westwood carded 74s, Tommy Fleetwood 76 and Phil Mickelson a 77 on a testing day.
Defending champion Francesco Molinari had earlier withdrawn from the defence of his title with a back injury.
Molinari carded a brilliant closing 64 at Bay Hill last year to claim a two-shot victory, but the winner of the 2018 Open Championship has not recorded a single top-10 finish since a tie for fifth in the Masters last April.
The 37-year-old Italian, who held a two-shot lead at Augusta National before finding water twice in the last seven holes, has slipped from a career-high of fifth to 26th in the world rankings.
In his pre-tournament press conference on Tuesday, Molinari admitted he was “a little behind” where he wanted to be at this stage of the season after three missed cuts and a tie for 53rd in the 72-man WGC-Mexico Championship, which does not have a halfway cut.
“The start of the season has not been as good as I hoped for, but it’s only the start,” said Molinari, who was replaced in the field at Bay Hill by South Korea’s Lee Kyoung-hoon.
“There’s been a few physical things and technical things but I’m working through it. I think I’ve got some good planning for the next few weeks building up to Augusta and some good direction to work towards.”
Molinari won all five of his matches in Europe’s Ryder Cup victory over the United States in Paris in 2018, but is currently 53rd in the qualifying standings for September’s contest at Whistling Straits.