The 22-year-old Northern Irishman, who became the tournament's first player to reach 13-under on the way to a 36-hole record of 131 on Friday, went one better on another overcast day at Congressional Country Club.
McIlroy took his total to 14-under 199 and surpassed the 54-hole mark of 200 set by American Jim Furyk in 2003 with another sensational display of shotmaking that improved his overnight advantage by two.
"I'm really happy with the way I played today and I just need to do that for 18 more holes," said McIlroy, who has held the lead in six of his last seven rounds in the Majors but is striving for a breakthrough victory.
A distant second was Yang Yong-eun of South Korea, who shot a one-under-par 70 for a total of six-under 207.
"I think it's a race for second place," Yang said through an interpreter. "At the same time, you never really know. He could shoot four-over and I could hit four-under."
Another shot back were world number two Lee Westwood (65), 23-year-old Jason Day of Australia (65) and American Robert Garrigus (68).
Tied on four-under par were Sweden's Fredrik Jacobson (66), who used a hot putter to reach five-under at one point, Spaniard Sergio Garcia (69) and American Matt Kuchar (69).
But the day belonged to McIlroy, who is determined to erase the memory of a final-round collapse two months ago at the Masters when he surrendered a four-shot lead with a disastrous 80.
McIlroy rolled in a 12-foot birdie at the fifth and tapped in for birdie at the par-five ninth before slipping to a bogey at the par-three 10th when he came out too strong from the front bunker.
The world number seven made up for that rare lapse with a 20-foot birdie at 11 and a seven-foot birdie at the 14th as Congressional's rain-softened greens remained receptive.
McIlroy, nearly flawless from tee to green in posting rounds of 65 and 66 over the first two days, was not so accurate off the tee Saturday but saved himself with precision iron play.
Finishing high in his follow-through and in perfect balance, McIlroy muscled shots out of the rough, blasted from bunkers with deft touch and putted with silky confidence as he continued to batter the US Open record book.
"He is one of the most phenomenal players I have ever seen," 2010 US Open champion Graeme McDowell said about his compatriot after shooting a 69 to move to level-par 213.
"What he is doing this week has been an accident waiting to happen."
McIlroy's eight-shot lead after 54 holes was second only to the 10-shot cushion enjoyed by Tiger Woods at the 2000 US Open at Pebble Beach.
Three-times Major winner Padraig Harrington of Ireland gushed about McIlroy's promise.
"Rory has proved in playing the Majors so far that he is comfortable making the scores," Harrington said.
"He's managed to lead after 18 holes, 36 holes and 54 holes and 63 holes so now all he has to do is get another nine holes and it looks like this will be the one.
"What is he, 22 years old? If you are going to talk about someone challenging Jack's (Nicklaus) record, there's your man."
When told of Harrington's comments regarding Nicklaus's record haul of 18 Majors, McIlroy shook his head.
"Oh, Paddy, Paddy, Paddy," said McIlroy, drawing laughs from the press corps.
"I'm still looking for my first one. That's all I can say. I'm looking for my first one."