BETHESDA, Maryland -- Rory McIlroy finally made a mistake, and even that wasn't enough to make this U.S. Open a fair fight.

For 17 holes Friday, McIlroy systematically dissected the monstrous layout at Congressional and put the same kind of hurt on a U.S. Open record book that doesn't change easily.

The 22-year-old from Northern Ireland made double bogey to finish the day but by then, the damage had been done. He shot 5-under-par 66 to head into the weekend at 11-under 131, setting the record for the best 36-hole score in the 111-year history of the tournament.

"To look at the day, the last two days, as an overall picture, it's been pretty good," McIlroy said. "I've played some of the best golf I've ever played. I'm converting a lot of my chances and not really putting myself in too much trouble, and that's really been the key."

His 36-hole score was one shot better than Ricky Barnes at Bethpage in 2009 and the gap would've been bigger had McIlroy not hooked his drive on 18 into the trees and knocked the recovery shot into the water left of the green. He made double-bogey there -- a bummer of a way to close a round that had been virtually error free up until then.

So good, in fact, that after back-to-back birdies on 16 and 17, McIlroy did something nobody had ever done. He reached 13-under par, the lowest score at any point in the history of the tournament, breaking a record held by Tiger Woods and Gil Morgan.

Despite the late hiccup, McIlroy still held an eight-shot lead over Y.E. Yang, who teed off at about the same time McIlroy finished.

Zach Johnson, Sergio Garcia, Robert Garrigus and Brandt Snedeker finished the day at 2 under for the tournament. They are, at this point, playing for second.

McIlroy, meanwhile, became the fastest player to reach double digits under par at the U.S. Open (26 holes). And he had a good chance of holding the biggest lead at the halfway point of the tournament, a record that belongs to Woods, who led by six after 36 holes of his indelible performance at Pebble Beach in 2000.

Coming off a final-round collapse at the Masters two months ago, McIlroy is still search of his first major. Now, it's just a matter of which way he'll go. Of the four previous players to reach double digits under par at the U.S. Open, two have won easily (Woods and Jim Furyk in 2003) and two have melted down (Morgan in 1992 and Barnes in 2009).

"It's a big challenge, but as you can see from my play, every time I get myself in position leading a major, I'm becoming more and more comfortable, and that's very important," McIlroy said. "I felt very much at ease today."

McIlroy holed out from the eighth fairway for an eagle that got him to 10 under. He made five birdies, 11 pars and still technically doesn't have a bogey, even though he did make the double on 18. He hit 15 more greens in regulation to bring his total to 32 of 36 for the tournament. Before No. 18, even when he got in trouble, he turned out OK. After hitting his approach into the bunker on No. 11, he saved par with a 10-foot putt.

But the signature shot from this day was the eagle on 8. Hitting a short approach, he bounced the ball at the back of the green, it hit twice then spun and rolled slowly before dropping into the cup. McIlroy raised both hands in the air. Phil Mickelson, playing in the same threesome, started applauding.

What else could he do?

"It's very near the best I can play," McIlroy said. "The second shot on the eighth, that was a bonus for that to go in. I just need to keep it going over the next couple of days and I'm halfway there, but there's still a long way to go."

McIlroy backed up the eagle with five straight pars, then stiffed his approach on 14 to four feet and made the putt to move to 11 under.

On No. 16, he barely missed an 8-foot eagle putt that would've put him at 13 under. But he got to that number on 17 with birdie following an equally close approach. No player had reached the magical 12-under point at any Open venue other than Pebble Beach, where Woods and Morgan both hit the mark.

Congressional is no Pebble, but neither is it turning into the beast it could've been. Soft greens and forgiving rough took some of the bite out of the 7,500-yard layout. McIlroy's first tee shot went slightly left into that rough but he had no problem wedging it out and onto the front of the green. His shot on 18 was so far left that the thickness of the rough wasn't the issue, but rather the angle he had to come in on.