Day carved out a steely three-under-par 69 to lead the Australian charge and sit just two shots behind first-round co-leaders Sam Burns and defending champion Rory McIlroy.
After staggering to the turn in even par, Day beat up Amen Corner with birdies at 12 and 13 before pulling off a contender for shot of the day on 14 to keep the momentum going on Thursday (Friday AEST).
"Oh wow, unreal J Day!" fans gasped after the one-time runner-up lasered an eight iron through half-a-dozen tree branches to set up a spectacular par.
"I had 153 yards to the pin. Wind was in out of the left. We had about three to four yards of spin with another seven-ish yards of hurt. So we are just trying to hit a big high cut," Day said.
"Sometimes in those scenarios it's coming out of first cut, if you contact wrong with it, it can fly straight through and you don't get the cut. Just comes out like a flyer, so it was important to slice across it."
Two clutch putts on 17 and 18 left Day delighted as the rest of the four-strong Australian contingent suffered mixed fortunes in the baking hot conditions at Augusta National.
"Definitely feels good when you hole those putts," he said.
"Obviously shooting three under is always better than shooting two under or one under.
"Need to make sure I'm staying on top of my speed because I feel like the greens are only going to firm out and get faster."
Predicting brutal conditions for the rest of the tournament, the former world No.1 was chuffed to have played himself into early contention but wary of getting excited.
"It'll be baked out and fast and guys will be kind of spewing on themselves out there. Be really difficult," Day said.
"So if they want, they can definitely soften it out if they need to. It'll be interesting. It's going to be a good challenge, regardless."
Day conceded McIlroy was already looking the player to beat, but said it would be dangerous to press too hard chasing the world No.2.
"McIlroy has got off to a great start. When I see five under leading, and with what weather we've got coming up, I've just got to be very patient," he said.
"Just got to take my birdies when I can and just don't try and force anything too much. We've got three more days. Just got to be very patient.
"Obviously I'm looking at Rory right now because the leaderboard is staring me right in the face. He's off to a great start defending the championship, but there are plenty of great golfers out there that you have to worry about as well. Not just him.
"And first and foremost, you've got to worry about yourself."
Former champion Adam Scott looked set to finish among the few players in red numbers before making a mess of the last in a level-par round of 72.
Cameron Smith defiantly claimed "I'm definitely not out of the tournament" after bogeying the last two holes in a two-over 74.
But Min Woo Lee faces a desperate battle to make the halfway cut with a 78.
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