Meanwhile the relatively short Brian Harman co-leads the tournament at the halfway stage.
That is the lowest score in relation to par in the 122 year history of the US Open, and it allowed Thomas to come hurtling out of the pack to set an imposing 54-hole target as he walked into the clubhouse at 11 under.
"Yeah, 12 under, I'd have about a 10-shot lead in most Opens", he said.
A women's advocacy group will protest the USGA's decision to hold this year's U.S. Women's Open at a Donald Trump-owned golf course by flying a banner over the men's U.S. Open on Saturday in Erin, Wisconsin.
Rickie Fowler, the Round 1 leader, shot a 1-over 73 to drop to 6 under par in a three-way tie for fifth with Jamie Lovemark and J.B. Holmes.
Four players are tied for the lead at the U.S. Open after Friday's second round.
Fowler, sharing a house with Thomas this week, was being left behind until he ran off three straight birdies and shot 68. He was 10 under, only two shots behind.
"Obviously, I'm not here on the weekend, so it's unfortunate", Day said. I'm looking forward to it very much. I've got four weeks off now. But he missed his first cut at a major since the 2012 PGA Championship.
Thomas finished in style, and it had nothing to do with his hot pink trousers.
He completed his record-breaking score by smashing his second shot to eight feet on the 667-yard closing hole, setting up an eagle which brought a roar that could be heard all over Wisconsin.
"I've shown what can happen with one bad swing".
He might need one at this rate. He loved Erin Hills, a course with wide fairways between the knee-high fescue that he figured would allow him to attack.
Shortly after Thomas finished off his record round, Paul Azinger said on the broadcast what many might now be thinking: "I'm scared of Justin Thomas".
"It's not every day you enjoy a round of golf with an eight on the card", Casey said.
"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.
"I was blown away by the support that I got today and this week", he said.
American golfer Justin Thomas.
The way this third round is going, odds are against him becoming the first player since Tiger Woods at Torrey Pines in 2008 to win a U.S. Open in his home state. The runner-up finish, along with a $1 million check, moved him into the top 50 and secured his spot in the Masters and U.S. Open. Thomas's ball landed softly on the green and settled eight feet right of the hole. PGA Tour rookie Xander Schauffele didn't make a bogey at Erin Hills until his 27th hole, had his hiccup with a double bogey on the par-3 13th hole but still managed a 73.
Despite a flurry of late birdies it was not enough to turnaround an erratic round littered with errors and missed opportunities. This is only his eighth major, and the third time he'll be playing on the weekend.
"But tomorrow the wind is going to come out of the north, it's going to blow, and the scores could be a good deal higher, if the weather comes to what they're predicting".
But the work is not done.
McIlroy, who has struggled through a nagging rib injury that has limited him to six tournaments this year, saw some positive signs in the second round after complaining about feeling rusty after his substandard opening 18 holes.
The Florida native has more good finishes in the majors than any of the other four.