Masters champion Jordan Spieth kept his bid to make history firmly on track as Tiger Woods missed just his fifth cut in 68 majors as a professional on Friday.
Spieth is looking to become only the sixth man, after Craig Wood, Ben Hogan, Arnold Palmer, Jack Nicklaus and Woods, to win the Masters and US Open in the same year.
The world number two can also become the first player since Gene Sarazen in 1922 to win multiple majors aged 21 or younger at Chambers Bay and set a clubhouse target of five under par that only Ryder Cup partner Patrick Reed was able to match.
Reed and Dustin Johnson both reached seven under par before dropping shots in the tougher afternoon conditions, with Johnson carding three bogeys in his last five holes to finish four under par alongside South African Branden Grace.
Starting on the back nine, Spieth birdied the 10th, 14th, 15th and 17th - he also three-putted the driveable 12th for par after missing from two feet - before running up a double-bogey six on the par-four 18th, which played as a par five in the first round and par four in the second.
"I think 18 as a par four doesn't make much sense," Spieth said. " You can hit it down the left centre of the fairway and still end up in the right bunker in trouble.
"There's a group of about 10, 12 guys that can fly it 310 yards that have an entirely different hole to play there. For anybody else you have to hit it in a five or six-yard area.
"So all in all I thought it was a dumb hole today, but I think we're going to play it from there again, so I've got to get over that."
Spieth led from start to finish when winning the Masters in April, setting new 36 and 54-hole scoring records and becoming the first player ever to reach 19 under par at Augusta.
"I'll probably draw a significant amount off it (although) it's playing different and I'm in a very different position," he added. "I'm not going to have a four, five-shot lead.
"I know that it's going to get tougher and tougher now that Saturday and Sunday hits. So I'll draw some on Augusta, but at the same time my patience level has to be even that much higher. I'm not quite putting myself in the same positions off the tee, so I've got to be a little more methodical.
"At Augusta I was kind of finding fairways, hitting it on the green and I was making everything. That would be nice here if I could do that, but it's a harder golf course than the Masters played this year."
As rookies, Spieth and Reed combined to win two-and-a-half points from three matches in the Ryder Cup at Gleneagles and also contested a play-off for the Valspar Championship earlier this year, won by Spieth.
" It's going to be fun," Reed said. "Any time I play with Jordan I enjoy it. The main thing for me is to stay focused and focus on what I need to do to play some good golf and plot my way around this course, especially if it gets firmer and faster and knowing where to leave the ball.
" I actually felt it was a pretty disappointing round to have six bogeys and I didn't get up and down once. I hit the ball in the middle of the green on 18, have no chance to putt a normal putt and stop near the hole and have to play Mickey Mouse golf to try to make par.
"Unfortunately it's a bad way to end the day but we're in a good position and we hopefully can have a good weekend and have a chance to win."
At the other end of the leaderboard, Woods added a second round of 76 to his opening 80 - his worst score ever in the US Open and third in the 80s in six events - to finish 16 over par and in a tie for 150th.
" I hit a little bit better today," said the former world number one, who won the last of his 14 major titles in the 2008 US Open. "But again I made nothing today. I didn't make any putts the first two days.
"On a golf course like this you get exposed and you have to be precise and dialled in. And obviously I didn't have that. Obviously I need to get a little better for the British Open and I'll keep working at it."