The Northern Irishman (13 under) was one-under-par through an eventful front nine on Sunday at Augusta National, four shots clear of Bryson DeChambeau in their blockbuster final-round pairing.
Ludvig Aberg and Justin Rose ensured it's no two-horse race, the pair surging to nine under to be equal with DeChambeau midway through their rounds.
Jason Day had birdie putts lip out and then just roll by the cup but still found two shots in a precision front-nine to remain in the picture at seven under through 10 holes.
Canada's Corey Conners (eight under), 2018 champion Patrick Reed (eight under) and world No.1 and defending champion Scottie Scheffler (seven under) had also made up ground on the leader.
McIlroy is in pursuit of a career grand slam that's beckoned since he won his fourth major as a 25-year-old 11 years ago.
It would be the first career grand slam, and sixth in golf history, since Tiger Woods completed the set at his first attempt with a dominant 2000 British Open title.
Near misses have haunted McIlroy though, as recently as last year's US Open when he coughed up a two-shot lead and DeChambeau snatched the title.
As a fresh-faced 21-year-old, McIlroy led by four entering the final round at Augusta in 2011, but shot 80.
DeChambeau and McIlroy had exited the clubhouse for their first pairing in a major like heavyweight boxers in a world title fight.
And it was shaping as a first-round knockout to the American after McIlroy found the bunker with his drive off the first tee, then three-putted to immediately cancel out his overnight two-shot advantage.
A birdie on the second gave DeChambeau the outright lead but a staggering McIlroy stayed on his feet, a two-shot swing on the third giving him back the lead.
McIlroy birdied the fourth and DeChambeau missed his mid-range putt for par and suddenly the buffer was three.
After the early exchanges both men scrambled towards the turn as the contenders queued, McIlroy's birdie and DeChambeau's miss from a similar spot creating breathing space.
Australia's other cut survivor Min Woo Lee (six over) dropped two shots in an eventful Sunday 74 that finished with a chip-in from the 18th bunker.
Adam Scott, Cameron Smith and Cam Davis all missed the weekend.
GOLF'S CAREER GRAND SLAM WINNERS
* Gene Sarazen - US Open (1922), PGA Championship (1922), The Open Championship (1932), Masters (1935)
* Ben Hogan - PGA Championship (1946), US Open (1948), Masters (1951), The Open Championship (1953)
* Gary Player - The Open Championship (1959), Masters (1961), PGA Championship (1962), US Open (1965)
* Jack Nicklaus - US Open (1962), Masters (1963), PGA Championship (1963), The Open Championship (1966)
* Tiger Woods - Masters (1997), PGA Championship (1999), US Open (2000), Open Championship (2000)
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