Reigning U.S. Open champion Bryson DeChambeau admitted he "wasn't fully on my game" after a mixed start to the tournament.

DeChambeau, like Rory McIlroy, was one of those to struggle on the opening day of the competition at Oakmont, with both making strong starts.

However, both endured collapses on the last nine, finishing over par, with work to do if they want to catch clubhouse leader JJ Spaun, who shot a four-under 66.

DeChambeau had been level par after 11 holes, but a run of three bogeys in five saw him post a three-over 73.

McIlroy, meanwhile, was two under through nine holes, but unravelled with four bogeys and a double bogey to card a four-over 74, before refusing to speak to the media.

DeChambeau, who is looking to become the fourth golfer since World War Two to win back-to-back U.S. Open titles after Ben Hogan, Curtis Strange and Brooks Koepka, was disappointed with how he finished his round.

"Just didn't get anything out of it," DeChambeau told reporters.

"I kept it one-under through seven, through eight, hit a perfect drive on nine, and this golf course can come up and get you pretty quick.

"You've just got to be on your game, and it got me, and I wasn't fully on my game. Pretty disappointed with how I played.

"It's not too far off. [I have just] got to get the putting a little bit more dialled and I'll be right there because three-over could have easily been two-under today."

His playing partner, Xander Schauffele, did not endure as difficult a round as his compatriot, and birdied the final two holes to pull back to two-under.

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It was a much stronger first day for Spaun, who was the only player in the morning wave to complete a bogey-free round.

In doing so, he equalled the best start to a U.S. Open at Oakmont and carded the first bogey-free round of his major career.

Only five players among the early starters finished under par, with Thriston Lawrence one shot back, and Spaun acknowledged that having never played at Oakmont before may have worked in his favour.

"I kind of came out here with no prior history at Oakmont, not really knowing what to expect even U.S. Open-wise," Spaun admitted.

"This is only my second one. I don't know if that freed me up in any aspect, but I just tried to take what the course gave me.

"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. I'm just overly pleased with how I started!"

World number one Scottie Scheffler was among the late starters, though he was not among the leaders at the halfway stage, while Im Sung-jae fell to joint-first after his first bogey on the 12th hole took him to four-under.

Patrick Reed also hit just the fourth albatross in U.S. Open history early in his round, making a two on the par-five fourth hole.