Louis Oosthuizen delivered a masterful third-round display to take control of the Deutsche Bank Championship in Norton, Massachusetts on Sunday to put the South African on the cusp of his first win in the U.S.
The South African shot a super 63 as he reeled off nine birdies, including a tournament record of seven in a row from the fourth, to move to 19 under par.
That birdie blitz enabled the 2010 British Open champion to move three shots clear of world number one McIlroy, who hit 67, with Tiger Woods (68) tied for third with fellow American Dustin Johnson (65).
"Probably the start anyone would dream of on that front nine," said Oosthuizen, who once shot a round of 57 at a course near his birthplace of Mossel Bay, South Africa.
"I made everything, so you get those days where you just look at a putt and you hole it."
One of those nine birdies included a 42-footer on the eighth as the 2012 Masters runner-up secured a tournament-record 29 on the front nine at TPC Boston.
"I've put myself in great spots this year, had a good chance at Houston, had a good chance at Augusta, a few other tournaments, WGC earlier," added Oosthuizen, referring to how close he has come to winning in the U.S.
"I'm just going to keep on doing what I'm doing. The more I'm in the position on the back nine on Sunday, or in this case on a Monday, then it's going to happen sooner or later.
Oostuizen's playing partner McIlroy, who had led by one stroke at the start of Sunday, had nothing but praise for the South African's round.
"He's very explosive, and he didn't really put a foot wrong," said McIlroy.