Rory McIlroy found Oakmont Country Club a bruising challenge as only five players from round one's morning wave ended under par at the US Open.
Masters champion McIlroy had started well and was two under through nine holes, but then unravelled with four bogeys and a double bogey as he posted a four-over 74.
JJ Spaun, who McIlroy beat in a play-off to win The Players Championship in March, leads the way on four under. The American had four birdies in his opening eight holes and his is the only bogey-free round halfway through day one.
South Africa's Thriston Lawrence is one behind Spaun after taking 67 shots on the notoriously difficult Pennsylvania course which is hosting a record-extending 10th US Open.
South Korea's Kim Si-woo is at two under, while Belgium's Thomas Detry, who was three under after nine, bogeyed his final hole to end on one under, alongside American Ben Griffin.
Scotland's Robert MacIntyre also bogeyed the 18th but a level-par 70 will keep him towards the top of the leaderboard heading into Friday's second round.
Defending champion Bryson DeChambeau was level par after 11 holes, but a run of three bogeys in five saw him post a 73. His playing partner Xander Schauffele - the world number three - birdied the final two holes as he shot a 72.
World number one Scottie Scheffler is among the players heading out in the later wave.
Last time the US Open was held here in 2016, only four players broke par and in 2007, the winning score by Angel Cabrera was at five over.
While not quite that penal yet, this tournament does look like living up to its self-proclaimed billing as "the toughest test in golf".
There were more than double the number of bogeys to birdies on the first morning and, with the course drying out, greens getting faster and the wind yet to bare its teeth, it promises to get harder.
World number two McIlroy has been in indifferent form since completing the career Grand Slam at Augusta National in April but serenely navigated the first half of his round.
He also struck the longest drive of his season, a mammoth 392 yards at the 12th (his third).
But Oakmont bit back on his second nine.
Driving the ball into the five-inch deep rough off the par-five fourth fairway, he needed three shots to escape and only a remarkable 30-foot putt limited the damage to a bogey.
Further shots went at his 15th and 16th holes before a first taste of the behemoth par-three eighth - playing at 276 yards - cost him a double bogey.
McIlroy has finished runner-up in the past two US Opens – in heartbreaking fashion last year after missing two short putts when leading in the closing stages – and has finished in the top 10 for six straight appearances.
While Thursday's round is far from terminal to his chances of winning a second US Open title, improvement is required in round two to keep him in the hunt at a course where he missed the cut when Oakmont last hosted this championship in 2016.
More to follow.