The 22-year-old, the son of multiple tennis Grand Slam doubles champion Liz Smylie, is two clear of 2022 British Open winner Smith and former world No.1 Day.
Smylie was given his first name decades after his father sat in the front row with tennis icon John McEnroe during an Elvis Presley concert in the 1970s.
He sprinkled his own stardust on day one of the Australian PGA at Royal Queensland in Brisbane -- the opening round of the DP World Tour's 2025 season.
At the par-5 ninth, his last, he recovered spectacularly from a poor bunker shot by holing a subsequent shot from an adjoining sand trap for birdie.
Smylie's eight birdies and two bogeys put him at six-under-par, one ahead of unheralded Swiss golfer Joel Girrbach, Australia's Matias Sanchez and Frenchman Victor Perez.
"Technically, I didn't touch the green and made (birdie), which is a good way to finish and there were lots of positives today," Smylie said about not needing a putter on the last.
"I've played a lot of golf (at Royal Queensland) and every part of my game's really good at the moment."
Perez plans to embrace forecasted heavy rain in pursuit of his fourth career DP World Tour win.
"It's a lot easier when you’re playing well," he said of the deluge.
"These conditions can create good separation and I was hitting the ball nicely, so I was able to free flow."
Left-handed Smylie upstaged Smith and Day, who joined crowd favourite and defending champion Min Woo Lee in a marquee group in the morning wave.
Smith and Day shot 67 to join a large group at four-under including LIV golfer Marc Leishman.
Three-time Australian PGA winner Smith was stagnant for nine holes before a terrific approach into the first hole, his 10th, for the first of four back-nine birdies.
"Seeing that putt go in (provided) a little bit of confidence and from there, when I'm putting good, the rest of my game frees up and I can really start to play some aggressive golf," Smith said.
Lee was five-under through 12 holes but dropped three shots and carded a three-under 68.
Day, a 13-time PGA Tour winner, played his first competitive round in Australia in seven years and was embraced warmly.
"It has been a while since I've been here so I wasn't not too sure how things would unfold but the Australian crowds are always great," he said.
