Daniel Berger hit an opening round of nine under to take a three-shot lead in the Arnold Palmer Invitational.
The 32-year-old has suffered a run of injuries – including a back issue which sidelined him for 18 months – and fractured a finger at the BMW Championship in August last year.
But having returned to full fitness, the American made six birdies on the back nine on his way to shooting a 63 – one short of the Bay Hill course record.
“I think the biggest thing for me is just getting in a bit of a rhythm,” said Berger. “When you come back [from injury], you are just kind of getting back into the flow of things and getting into your routine.
“I have really played not that poorly, I just haven’t had the results. You just kind of keep doing the same things and good things will come, so that’s kind of where I feel like I am at.”
A double bogey on the 13th hole, followed by dropped shots on the 14th and 18th, proved costly for Northern Ireland’s Rory McIlroy as he had to settle for a level-par round.
World number one Scottie Scheffler is at two under, along with England’s Justin Rose.
Sweden’s Ludvig Aaberg sunk a long eagle putt on 12 as a strong finish left him three shots behind Berger on six under.
Former Open champion Collin Morikawa also hit a 66 to stand a shot clear of his fellow American Cameron Young and Venezuela’s Jhonattan Vegas.
