Benign conditions on Thursday made for a feast of low-scoring, with a record 44 players dipping under par - the most ever in the first round of the US Open. His 7-under par tied the record to par for the first round of a U.S. Open held by Jack Nicklaus and Tom Weiskopf, who each shot 7-under 63 at Baltusrol in 1980.
It opened the door for England veteran Paul Casey (71) to share honours atop a no-frills leaderboard with countryman Tommy Fleetwood, Brooks Koepka and Brian Harman (all 70) at seven-under after the second round at Erin Hills.
In the final round at Oakmont a year ago, his golf ball moved slightly as he positioned himself for a short putt on the fifth hole.
T5. J.B. Holmes (-6): For every bogey on the day, Holmes had an answer with a birdie on the subsequent hole.
And despite carding four birdies in his last six holes for a 71 on Friday, the four-time major victor saw his own pre-event prediction come true, having said on Tuesday that anyone who could not hit the wide fairways "might as well pack your bags and go home".
The U.S. Open was only his sixth start this year.
Shane Lowry is the only Irish golfer now inside the projected cut mark of 1-over at the US Open as Day 2 unfolds.
Another youngster looking for his first major win, Patrick Reed, is listed at 20/1 after shooting a four-under 68 in Round 1. "I have a tee time on Saturday (and) so you just kind of learn along the way to never really give up".
"I made the mistake of leaving it short and having it come back down to my feet", he said.
With the afternoon wave several hours away, the early projected cut-line for the top 60 players and ties is one-over.
"Even though it's very disappointing to not be here on the weekend, I think these last two rounds will serve me well going into the summer", McIlroy said.
"Showed up for the last six holes, anyway", said the four-times major victor, who made four birdies on his second nine, including his finishing hole, the par-three ninth.
Lowry had a poor front nine, which featured four bogeys and a birdie.
"It's been the best preparation going into a major, I felt like, in my career, which is playing in majors and knowing what I needed to do to prep for a tournament such as this", Day said.
His problems started when he tried to lay up and round the right rough.
But then, starting at 17, he reeled off five straight birdies with putts of 13 feet, 7, 3, 11 and 10.
"I couldn't get comfortable over the putter".
"I played great", said the 44-year-old, who shot 80 when playing with eventual victor Johnson in the final round at Oakmont. "There's no secret that I'm very fiery and a competitor and I've let my emotions get the best of me for sure, so I'm understanding that if I can control those I certainly have a better chance of playing well".