Spaun defied Oakmont’s fearsome reputation by conjuring a bogey-free, four-under par 66 – his lowest round in nine major championship starts.
The 34-year-old American equaled the lowest first round of any US Open at Oakmont, a 66 by American Andrew Landry in 2016, producing only the eighth bogey-free US Open round at the Pennsylvania club hosting for a record 10th time.
“I’m just overly pleased with how I started,” Spaun said. “I hit a lot of good shots and tried to capitalise on any birdie opportunities, which aren’t very many out here.”
“But I scrambled really well, too, which is a huge component to playing well at a US Open, let alone shoot a bogey-free round,” he added.
Spaun started on the back nine, birdied four of his first eight holes, then parred his way to the clubhouse, his first-nine 31 a best for an Oakmont US Open.
“This is probably one of my better putting rounds all season,” Spaun said.
He salvaged pars after finding the Church Pew bunker at the fourth hole and a bunker at the sixth.
“Those are huge, especially at a US Open, to keep your round going and not go the other way,” Spaun said.
South African Thriston Lawrence was second on 67 with South Koreans Kim Si-woo and Im Sung-jae on 68 with five-time major winner Brooks Kopeka.
“I definitely think it was gettable,” Koepka said of the course.
American Koepka sank a 42-foot eagle putt at the fourth, made bogeys at the 10th and 14th holes then closed with back-to-back birdies from five feet at 17 and 16 feet at 18.
“I played pretty consistent, drove it really well,” Koepka said. “Iron play was pretty good. When I did miss it, I felt like I missed it in the correct spots.”
World number one Scheffler, trying to win back-to-back majors after taking the PGA Championship in May, endured a roller-coaster round of 73 with three birdies and six bogeys – his worst opening major round to par.
“I made some silly mistakes out there,” Scheffler said. “But at the same time, I made some key putts and some good momentum saves – overall just need to be a little sharper.”
Scheffler, whose nine wins last year included Paris Olympic gold, has won three of his past four starts.
Second-ranked McIlroy, who completed a career Grand Slam by winning the Masters, shot 74. The back-nine starter birdied 11 and 12 but made four bogeys and a double bogey on his second nine.
Defending champion Bryson DeChambeau shot 73 with five bogeys and two birdies.
“Pretty disappointed with how I played,” DeChambeau said.
Six-time US Open runner-up Phil Mickelson, who turns 55 on Monday, opened with a 74 in his bid to complete a career Grand Slam.
Im briefly had the solo lead at five-under but three bogeys in his last seven holes derailed him.
“I played five-under but ended up at two-under,” Im said. “I planned to shoot under par, so I made the goal.”
Two-time major winner Jon Rahm was in a pack on 69.
“I’m extremely happy,” the Spaniard said. “I played some incredible golf to shoot one-under.”
American Patrick Reed made the fourth albatross in US Open history with a three-wood from the fairway from 286 yards on the fourth hole – the first at any major since Nick Watney in the 2012 US Open at Olympic Club.
“Just happened to be a perfect spot, perfect club,” Reed said. “For it to go obviously is a bonus.”
Ireland’s Shane Lowry holed out from 160 yards out in the fairway for the first US Open eagle at Oakmont’s third hole, but it was his lone bright spot in shooting 79.
American Maxwell Moldovan made the first US Open eagle at the first, holing out from the fairway from 189 yards, then looking to the heavens with a smile. He fired a 76. (AFP)