Previous longest was 7,695 yards, during the second round of the 2015 U.S. Open at Chambers Bay, won by Spieth.
Harman closed with two birdies and three tough pars for a 5-under 67 that gave him a one-shot lead at 12-under 204.
After bogeying three straight holes on his back nine, his first dropped shots of the tournament, Fowler limped into the clubhouse with a 73 to fall to six under for the tournament.
Oosthuizen, whose sole major title to date came at St Andrews in 2010, birdied the first, fourth, seventh and eighth to move to four under par, while 2015 victor Spieth had covered his first 11 holes in level par.
Players have voiced their opinions on the difficulty of Erin Hills, but with the weather holding out and relatively mild winds Thursday, players called the course "very fair." 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. He hit 12 of the 14 fairways in the second round, and one of the misses came on No. 15, when he was in desperation mode and tried to drive the green on the 357-yard hole. "It means I have a lot better chance to win the tournament than I did when the day started". You've still got to make a score.
The last thing the USGA wants is to trick up the golf course, because the U.S. Open has had too many bumps (conditions at Chambers Bay, the rules mess at Oakmont) to risk anything but a smooth week.
Louis Oosthuizen is making the biggest move the morning in the U.S. Open at Erin Hills.
US Open sees top seeds cut, four-way tie for lead
The previous lowest score in relation to par was the eight-under 63 shot by Johnny Miller in the final round of the 1973 Open. An 11-year-old golf course shaped from Wisconsin pastureland was sure to bring something different to the U.S.
"But you never really know until you put a card in your hand and you're under the gun a little bit". It's tied (with the par-3 fourth hole) for the hardest on the course in relation to par, averaging a score of 3.273.
"Yeah, it's a good spot to be in", he said.
American Justin Thomas equalled the lowest score in major championship history in stunning fashion to surge into contention for the US Open at Erin Hills.
Day added a 75 to his first-round 79, which included two triple-bogeys.
Another possible factor could be the course, the first par-72 used for the U.S. Open since 1992. I think the more rounds I can play, I'm hopefully going to get rid of all that stuff and hopefully strip it down to what you saw the last six holes. It's the first 63 at the U.S. Open since Vijay Singh in 2003 at Olympia Fields and the fifth overall in tournament history. "I think you've got a lot of great players on top of that leaderboard and you've got some guys that hit it forward, some guys that putt it great, some guys that do everything pretty good". "I was just happy that I was the one that was able to take advantage of it today".
"The majors have a different feel and sound to the roars and to hear the crowd go insane when I holed that putt on 18 was really cool", said Thomas, whose back-to-back wins in Hawaii in January included an opening 59 in the Sony Open.
Day and McIlroy, just to name a few, might not make it that far.
But Erin Hills ended up being the proverbial good walk spoiled for the top players in the world.
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