In-form Scott carded a first-round eight-under-par 63 to be tied second with China's Zecheng Dou, after the first round at TPC Craig Ranch.
He trails pacesetter S.Y. Noh by three shots after the Korean's dazzling and flawless round of 60 featured nine birdies and an eagle on the par-5 ninth.
Day is a stroke behind Scott in a six-way share of fourth following a bogey-free seven-under 64.
Making his PGA Tour debut, Micheluzzi posted a 65 to be tied for 10th following a spectacular finish to his round.
The PGA Tour of Australasia Order of Merit winner was plodding along at one over par through his first seven holes.
But he exploded to life with a hole-out wedge shot from the fairway for an eagle two on the par-4 12th either side of five birdies in last 11 holes.
"It's probably something I'll never forget," Micheluzzi said.
"Just the run I went on as well, which I have been doing that of late in tournaments back home and then one in Korea as well, where once I got running, like nothing could stop me, which was cool.
"Just got to work out all the little things like the little missed putts here and there. But, yeah, I'm just excited."
Micheluzzi had said pre-tournament he wanted to "plot along and shoot four, six under" and he delivered in style.
The 26-year-old Melburnian will also make his major debut at next week's PGA Championship in New York and Thursday's round should serve as a huge confidence booster.
Scott registered his first top-10 finish of the year at last week's Wells Fargo Championship and the former world No.1 picked up from where he left at Quail Hollow.
He accrued nine birdies, his only bogey coming after an uncharacteristic poor approach from 100 yards on the par-4 sixth hole.
"All really good solid stuff, especially off the tee," Scott said.
"It's probably the best I've driven it in forever. That was a nice way to start every hole. Took advantage of that nicely."
Day produced a brilliant round of shot making for his seven under, but only got scoring when his putter started running hot around the turn.
Playing in the marquee group with Scottie Scheffler, Day gifted his fellow former world No.1 a six-shot start through six holes but wound up signing for the same score of 64 by round's end.
Day birdied four consecutive holes from the eighth, picked up two more shots on 15 and 16 before icing his round with a 24-foot birdie putt at the last.
"It's kind of hard to be patient sometimes when you're watching your playing partner shoot six under through six, playing some pretty top-notch golf," Day said.
"Sometimes you've just got to admire it, but it was good to get a four-under run in the middle part of my round.
"I think maybe 20 under or so will win this, so you're thinking at least five a day would be good, and it was nice to get a couple more than that."
Day also shares fourth spot with Americans Luke List, Richy Werenski, Scott Piercy and Ryan Palmer.
Min Woo Lee, Harrison Endycott, Greg Chalmers and Cameron Percy, all at two-under 69, are the next best Australians in a tie for 65th.
Geoff Ogilvy posted a 70 and Aaron Baddeley an even-par 71.
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