Day rolled back the years with a vintage display of scrambling to eke out a gritty one-under 71 to climb to four under for the championship after Friday's second round.
But while the former world No.1 is tied for seventh and on track for a sixth top-10 finish at Augusta National, any realistic prospect of finally landing an elusive green jacket appear forlorn.
With six birdies in his last seven holes, McIlroy surged from one shot in front, after Day birdied the 14th hole, to eight clear of the former PGA Championship winner.
Day, though, is promising to hang tough in pursuit of a now most improbable triumph.
"Not many people are obviously talking about me, which is good," Day said.
"I'm just going to go under the radar and do my thing. I felt like it's been a pretty good formula so far over the last few days, what I've done.
"I feel confident in my ability to be able to shoot a good score. I've been driving it nice. Been putting myself in the correct positions when I do miss it.
"So the experience of being here for the last 15 years is definitely helpful."
McIlroy will carry a record six-stroke lead over American pair Sam Burns and 2018 champion Patrick Reed into the weekend.
But Day knows anything can happen.
"The weekend at Augusta is a different beast, obviously," said the 2011 runner-up.
"Getting through the cut line and playing good is one thing. Then once you get to the weekend, things open up and scoring gets really tight.
"Unless someone runs away with it, typically scoring gets tight. So just be patient."
While Day is clinging to hope of a miracle comeback, 2013 champion Adam Scott fell 14 shots adrift of McIlroy at two over with a second-round 74.
While Scott at least made the cut for a 22nd time in 25 attempts, Australian No.1 Min Woo Lee and fallen star Cameron Smith both crashed out of the tournament.
Smith was too gutted to speak to waiting media after his second-round 77 left him at seven over and three shots short of the cut line.
In a shocking run, the one-time world No.2 has now missed the cut at the past six majors, but Day said it would be foolish to write him off.
"Cammy, he's such a good golfer. The guy won the British Open, won The Players (Championship). He's an unreal golfer," Day said.
"He has a tremendous short game and really good wedge game.
"So it's unfortunate this golf course actually should suit him really well, just of how good his game is. He's a great putter, too.
"He might be going through a bit of a tough run right now, especially during the majors, but that's golf. I mean, who knows, he could win the next major and we're not thinking about it again."
Lee was even worse, rounds of 78-77 leaving him at 11 over, with only eight players finishing lower.
"Making two birdies out here in 36 holes is not going to cut it," Lee said.
"Didn't have my best at all. I don't know. I mean, the preparation was unbelievable. You would think I would be winning the tournament the way I was playing leading up to the event.
"Bummed that I couldn't get anything going."
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