This comes two weeks after he missed the cut at the Memorial Tournament.
He had struggled through a nagging rib injury that has limited him to six tournaments this year, but was buoyed by his second round at Erin Hills after complaining about feeling rusty after his substandard opening 18 holes.
He three-putted on No. 13.
"In college I normally am the longest".
"I couldn't possibly shoot any higher than I did", Johnson said.
McIlroy is back to even-par for the round after bogeying the first three holes on the front nine. A lot of them had to come from far back going into the weekend, and it was hard to put together two good rounds.
It's not much different than what USGA executive director Mike Davis said about bringing the U.S. Open to Erin Hills, a course that only opened 11 years ago.
Joining them were British Open champion Henrik Stenson (No. 6), Alex Noren (No. 8), Jon Rahm (No. 10), Justin Rose (No. 11) and Adam Scott (No. 12).
Masters champion Sergio Garcia returned a tidy one-under 71 to leave the Spaniard four back and in contention for a second straight major.More news: Rosenstein says he's seen no good cause to fire counsel
Day at least managed to find one positive spin. "Maybe a reachable par 4, should be firmer and faster, barring any rain", he said. Results are not what will make this a great week at Erin Hills for the 50-year-old Stricker. There no longer is a 10-shot rule that would allow anyone within 10 shots of the lead to make the cut.
Some of the world's top-ranked players and pre-tournament favourites look poised to miss the cut. "I took advantage of it the last few days". If only he had hit the fairways.
Fellow Australians Wade Ormsby (six-over after a second 75) and Nick Flanagan (10-over after a 79) also went home early, joining Johnson (four-over after a 73) and four-time major victor McIlroy (five-over after a 71). But he missed a pair of 10-foot birdie chances in that stretch, and one of the birdies he made was a two-putt from 20 feet on the par-5 seventh. "I probably could have birdied all of the last six", McIlroy said.
"Just to see their games and how they play, I've grown up watching them". He missed almost two months at the start of the year when he was diagnosed with a slight rib fracture, and then the latest break after The Players Championship.
It was only previous year at Oakmont that Andrew Landry, who had to make it through two stages of qualifying to get into his first U.S. Open, shared the 36-hole lead with Dustin Johnson.
"Rickie was up there (and) us three played pretty solid", Niebrugge said.
"Just excited to get on a run of golf and get going".
His schedule will pick up immediately. "I think the biggest thing is not getting ahead of yourself".
Among the early starters on another steamy day at Erin Hills, Casey survived a hair-raising round that included a gut-wrenching triple-bogey and a thrilling run of five straight birdies and ended with him signing for one-under 71.
"It was a good display - all my own fault - but a good display of what can happen if you get out of position on this golf course, which is what I did on 14", Casey said. "I guess it's rare you get through a U.S. Open or any major without some kind of a hiccup".