The 2015 PGA Championship winner made the most of his early tee time, collecting eight birdies before the wind started to pick up and make scoring more difficult for the late starters at The Australian Golf Club.
Playing the back nine first, Day dropped a shot at the par-three 15th hole, then double-bogeyed the eighth hole, his second last, to finish three strokes behind the early clubhouse leader Cameron Davis.
“I made a couple of mistakes out there with some swings, but made, I think, eight birdies, which was nice and finished at five-under, which is a good start to the week,” Day said.
“I think it would have been a little bit disappointing if I came out this morning and shot an even-par round. I think it’s definitely out there to shoot a deep one.
“That’s kind of the thing, you almost need to come out and capitalize when there’s no wind, because it’s very warm in the morning, the ball’s starting to go a lot further in the mornings and there’s no wind.”
Davis birdied six of his first seven holes before the wind gusts put a halt to his phenomenal start and he eventually signed for a 63, enough to grab a two-stroke lead over Taylor Macdonald, ranked 1,809th in the world.
Davis, who turned professional after winning the 2015 Australian Amateur title then helping Australia win the Eisenhower Trophy in 2016, could hardly contain his excitement after chipping in for birdie at the seventh to get to six-under.
The 22-year-old made his only bogey of the day at the ninth but regained his composure and played the back nine in three-under to claim the early outright lead.
“To be able to play the back nine without any bogeys, just keep on making a couple of birdies, I felt like that kind of shows that I was able to not get too far ahead of myself, which has definitely happened in the past,” Davis said.
“That’s one of the things I feel like is improving for me.”
The defending champion Jordan Spieth was among the late starters.