Rory McIlroy is enjoying the challenge of having his game tested to the limit by Merion after adding a battling level-par 70 to his opening 73.
The 2011 US Open champion carded identical scores to playing partner Tiger Woods as he hit back from a poor finish to his opening round.
McIlroy was level par after 11 when darkness ended play on Thursday, and he made a solid start on the resumption with three straight pars before he dropped three shots over the final four holes.
But after starting his second round at the 11th, he opened with a pair of birdies before giving one back at the 14th, and a careless bogey at the first dropped him to four over.
A birdie at three was offset by a scrappy six at the par-five next, and he made another mistake at nine to undo an excellent birdie at the eighth.
"It was solid. It felt like every time I made a birdie, I made a bogey straight away," he said. "It's a course you feel like you can shoot a score but because of the pins, it's tricky.
"But I'm there for the weekend and I'm happy about that. It tests every aspect of your game. You've got to drive it well, be very tactical. There were people talking about 63s and 64s at the start of the week but I never saw it at all.
"I'm very happy. Right in there for the weekend. I don't think I'll be too far away by the end of the day. I'm in a nice position going into the last two days.
"At the end of the day, there's going to be a guy lifting the trophy at the end of the week. It doesn't matter if he's plus five, minus five or plus 16."
But his fellow Northern Irishman Graeme McDowell crashed out after rounds of 76 and 77 left him outside the top 100 on 13 over par.
"That was what US Open golf was all about," said McDowell, whose double-bogey at the last was his seventh of the tournament. "I'm temporarily dejected.
"I'll shake it off and I'll get ready for the Open Championship in a few weeks. That's my next target, The Irish Open and the French Open before that.
"I'll be competitively sharp going into Muirfield and I'll continue to draw on this season."