The 26-year-old from Northern Ireland had 11 birdies with no bogeys to break his own Quail Hollow course record of 62.
"I feel like the best player in the world and I wanted to go out there and prove that," McIlroy said.
McIlroy's effort gave him an 18-under total of 198 and a four-shot lead over American Webb Simpson who shot a 68 and was alone at a 14-under 202.
McIlroy's hopes of reaching golf's magical number of 59 ended with pars at 17 and 18.
But the finish didn't mar a spectacular round that included streaks of four birdies in a row from the seventh through 10th holes and five birdies on the trot from the 12th through 16th.
Robert Streb (71) was in third, seven shots adrift while Brendan Steele (68) and Patrick Rodgers (70) were tied for fourth on 10-under 206.
Justin Thomas (65), Carlos Ortiz (66) and Will Mackenzie (70) were a further stroke back on 207.
The Quail Hollow course record of 62 was established by McIlroy in the final round in 2010 -- when he captured his first US PGA Tour title.
Zimbabwe's Brendon de Jonge matched the mark in the second round last year.
McIlroy said he knew from watching the morning scores that the course record was "gettable".
"But you never think that you're going to go out and do something like that," said McIlroy, who also set the tournament's 54-hole scoring record.
- Thinking '59' -
After opening his day with back-to-back pars, McIlroy birdied 11 of the next 14 holes.
Highlights included driving the green at the 346-yard par-four 14th en route to a birdie.
"I was thinking 59 from the 14th tee box," McIlroy admitted. Only six players have broken 60 on the US PGA Tour, but the idea seemed anything but farfetched after he rolled in a 10-foot birdie putt at 15 and a 14-footer at 16.
McIlroy said he was a tad disappointed with his seven-iron at the par-three 17th, where his tee shot left him 40 feet for birdie and he settled for a par.
"Obviously I'm delighted with the score, and it's nice to break your own course record as well."
McIlroy said he now needs to turn his attention to Sunday's final round when he will be seeking his 11th PGA Tour title.
"I guess I just have to forget about today. It's a whole new challenge tomorrow," said McIlroy, who has no plans to start playing it safe.
"Even though I've got a lead there's no reason why I shouldn't go out there and be aggressive and try and make as many birdies as I can," he said.
Five-time major champion Phil Mickelson, the 2010 runner-up, had climbed within one stroke of the lead thanks to four front-nine birdies.
But closed with a triple bogey at 18 to fall into a group of five players tied for ninth at eight-under.
source: www.abs-cbnnews.com
