McIlroy is facing the grim prospect of becoming the first - and only - man since Harry Cooper in 1936 to relinquish a halfway lead of at least five shots after another wild display of driving.
Ranking last in the field for driving off the tee this week, the defending champion's waywardness finally caught up with him in a one-over-par round of 73 on Saturday.
Bidding to follow legends Jack Nicklaus, Tiger Woods and Nick Faldo as only the fourth player ever to win back-to-back green jackets, McIlroy still holds a share of the third-round lead.
But suddenly six more players are within four strokes of the lead, jointly held by American Cameron Young, and another half-dozen within six, setting the stage for a final round for the ages.
"I didn't quite have it today," McIlroy said.
"The course was obviously gettable. There was a lot of good scores out there and the quality of the chasing pack is obvious.
"There were a lot of guys that shot good scores. There's a lot of guys in with a chance tomorrow.
"I'm still tied for the best score going into tomorrow so I can't forget that, but I do know I'm going to have to be better if I want to have a chance to win."
Young, the world No.3 and last month's Players Championship winner, fired the day's equal-low round, a sizzling seven-under 65.
With a watery double-bogey on the 11th hole amid a dreaded run through Amen Corner, then a bogey at No.17, McIlroy joined Young at 11 under.
Young's fellow American Sam Burns (68) is solo third at 10 under, with Ireland's 2019 British Open champion Shane Lowry (68) outright fourth a further shot back after etching his name in golf's history books.
With a magical hole-in-one at the par-3 sixth hole, Lowry became the first player ever to record two Masters aces, having also knocked one in on the iconic 16th a decade ago.
Lowry's latest heroics continue his penchant for aces on golf's grandest stages.
The former US Open champion also holed out at Augusta's iconic 16th hole in 2016 and aced the island 17th at TPC Sawgrass and the seventh at Pebble Beach.
Only three shots back, Day is right there at eight under.
The 38-year-old is sharing fifth place, somewhat fittingly with England's three-time runner-up Justin Rose (70) after carving out a 68, the best third round of his 15-year Masters career.
After finishing runner-up on debut at just 23 in 2011, Day and Rose - who lost out last year to McIlroy in an epic play-off - will both be trying to pen famous redemption stories while playing together in the third-last group on Sunday.
Rounding out a stunning star-studded leaderboard is world No.1 Scottie Scheffler, who earlier fired up to snare the clubhouse lead with his own sparkling 65.
And a day after "living in the toilet" with food poisoning, China's Haotong Li has somehow found himself in Masters contention instead.
Playing the front nine in five under, Lee helped helped himself to a third-round 69 to be joint seventh with Scheffler - just four shots back.
After opening his title defence with rounds of 67 and 65, McIlroy had built the biggest 36-hole lead in the Masters' 90-year history.
But the world No.2 was facing a restless Saturday night after relinquishing a six-stroke advantage, just as Norman did in the final round 30 years ago to gift a third green jacket to Nick Faldo.
