When the third round began, McIlroy had built a six-stroke lead, the largest lead ever after 36 holes in the Masters.
But after dream rounds - 67 and 65 respectively - in the first two rounds of the year's first major, the reigning champion had a rough Saturday at Augusta National.
Bogey straight away
McIlroy swung his putter on the first hole and made a bogey. Things got worse on hole 11, where he hit his shot into the water and made a double bogey.
The Northern Irishman wasn't completely out of contention, but after a 73 and -11 overall, he invited a host of competitors into the fight for the green jacket.
Scottie Scheffler went out a couple of hours before McIlroy and shot 65 - the world number one is only four strokes behind the world number two heading into Sunday.
American Cameron Young also shot 65 and shares the lead with McIlroy heading into Sunday.
Sweaty in the sun
A handful of other players have the chance to challenge for victory in what could be a highly dramatic finish in the demanding heat of Georgia.
Sam Burns (USA) played with Rory McIlroy on Saturday and narrowed the gap from six to one stroke.
Shane Lowry made a hole-in-one on hole 6 on the way to his round of 68, leaving Lowry two strokes behind the leading duo.
Englishman Justin Rose - three-time runner-up in the Masters - and Australian Jason Day are also in contention, three strokes off the lead.
Ludvig Åberg (tied 21st at -3) and Alex Norén (tied 39th at +1) are, however, too far back despite each shooting a round of 69 on Saturday.
Rory McIlroy took his long-awaited first victory in the Masters last year, a triumph that completed a "career grand slam" - victories in all four majors.
But only after a playoff, when he surrendered a lead on Sunday.
"I like to think I'm playing a little more freely now than I was on the leaderboard last year. And I'm going to play like I already have a green jacket, which I do. Sometimes I have to remind myself of that," McIlroy said.
Should he defend his title, he will become the fourth player - after Jack Nicklaus, Nick Faldo and Tiger Woods - to win the Masters two years in a row.
