Jason Day warms up on cold day at Arnold Palmer Invitational

In the first staging of the event since the September death of Palmer, Day adjusted to the conditions well enough to shoot two-under-par 70 on the Bay Hill course in Orlando.

While Day is using this week as part of his build-up for next month's Masters, Danny Willett's preparation for his title defence at Augusta National suffered a setback when he pulled out shortly before his tee time because of illness.

It was only a few degrees above freezing when Day teed off about an hour after sunrise, and he had to adjust quickly to the shorter ball carry with, he said, his mid-irons travelling about 20 yards less than usual.

"I don't play social golf in cold weather," Australian Day told Golf Channel.

Day used last year's Arnold Palmer Invitational as a springboard to a sizzling spring, the first of three victories in six starts.

Though he is still ranked second in the world behind Dustin Johnson, Day has not quite been firing on all cylinders so far this year.

"Hopefully I'll be ready by Augusta," he said.

"Golf is a funny game. You can go on spells where everything is ... not going the way you want it to and you’re thinking 'will I ever dig myself out of this hole'. And there are some times you’re going 'I can’t miss a putt, I can’t miss a drive, I’m playing the greatest golf of my career.'"

His 70 was matched among the morning starters by Americans Stewart Cink and Kevin Kisner and Italian Francesco Molinari.

Argentine Grillo recovered from two early bogeys, reeling off seven birdies to jump to a three-shot lead with his five-under 67.

The tournament started in a sombre mood in the absence of Palmer, one of golf's greatest players whose immense popularity drew a legion of fans to the game.

His passing was honored on Wednesday by a ceremonial succession of drives, led by Palmer's grandson, PGA Tour player Sam Saunders. (Reporting by Andrew Both in Cary, North Carolina; Editing by Frank Pingue)