The last time Day missed a cut in a major was the 2012 PGA Championship, just a month after his son was born.
Fifteen of the 18 holes had been cut within seven yards of the edge of the green and despite the 68-man field being separated by eight shots, only former Open champion Louis Oosthuizen had been able to move into the top 15 after nearly four hours' play.
Reed is tied with Charley Hoffman and Tommy Fleetwood for the lead at 8 under. Southpaws have won every major except the U.S. Open.
The 24-year-old Thomas eagled the par-5 18th for just the fifth 63 in U.S. Open history. His best finish came in his first start at the event in 2013, when he tied for 10th at Merion.
Prior to the tournament, the 28-year-old Northern Irishman had defended the severity of the rough, saying the fairways were "the widest played at a US Open" and "if we can't hit within that avenue, you might as well pack your bags and go home".
But not everyone left with a smile as the world's top six ranked players all failed to get into red numbers.
Casey, Koepka, Harman and Fleetwood have yet to tee off. The only disappointment was a three-putt bogey from 35 feet on the final hole. He shot a 69 and was at 2-under 214.
The little lefty from Georgia has a chance to make a little history at Erin Hills. He just has to listen to the crowd.
Day never had a chance, following his career-worst 79 in a U.S. Open with a 75. He's one shot back of the lead shared by four golfers who tee off later Saturday. The ball landed on the front of the green just soft enough that it rolled out 8 feet by the hole.
Hideki Matsuyama is nine shots back after shooting a 74.
Sure, he birdied four of his last six holes to salvage a 71 and finish at 5-over 149.More news: Arsenal midfielder Ramsey scores 'panenka' penalty in same stadium it was invented
One stroke back are a trio of Americans in J.B. Holmes, Jamie Lovemark and first-round leader Fowler. "They're the widest fairways we've ever played in a US Open".
More startling than the low scores was smoke rising from about a 800 meters away when a commercial blimp, not affiliated with the tournament, crashed into a field and burst into flames.
No one took advantage like Fowler. Early in the round, when the wind was up, the 9th was playing as the toughest hole on the course - tougher than the 249-yard par-3 6th, tougher even than the 525-yard (!) par-4 10th.
There's a four-way tie for the lead at the U.S. Open.
Another warm, dry day is drying out the course during the second round of the U.S. Open, and the players are noticing a difference.
Not bad for a guy who made the cut on the number.
"I was upset with the score I had made, but it did not have any effect on my attitude or how I was going to then approach the rest of the round or the next shot".
Dustin Johnson is 1 over, on the cut line with six holes left.
It was the first time since the world ranking began in 1986 that the top three players missed the cut in any major.
At most major championships, there are a few surprises - players who no one expected to make a run at a major title.