The revered track outside Pittsburgh became waterlogged as storms caused three separate weather delays before play was called off for the day.
Only nine players finished their first round and more than half the field were yet to start, leaving American qualifier Andrew Landry, the world No. 624, top of the leaderboard.
The 28-year-old was three under par playing the final hole, one shot clear of world No. 4 Bubba Watson and Danny Lee, when the hooter went for the final time at 1551 ET on Friday.
American amateur Scottie Scheffler shot 69 to finish one under alongside Kevin Streelman, Lee Westwood and Harris English.
Defending champion Jordan Spieth was one over after 11, with third-ranked 2011 winner Rory McIlroy four over alongside Masters champion Danny Willett.
World No. 5 Rickie Fowler had a difficult day and was six over after 12 at the end.
During the first delay, several players, including England's Westwood and Luke Donald and Germany's Martin Kaymer, sought refuge in the media center to watch England's 2-1 win against Wales
in the Euro 2016 soccer tournament.
"It's obviously a frustrating day having to keep coming off, but there's nothing you can do about the weather," said Westwood, who eagled the par-four 14th.
Among those still to start was Phil Mickelson, celebrating his 46th birthday, who is seeking a first U.S. Open title after a record six runner-up spots.
Victory in his national championship would make the popular left-hander the sixth player to win the career grand slam of all four major titles.
Only Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus and Tiger Woods have achieved the feat, and Mickelson admitted earlier this week it plays on his mind.
"This is the tournament I want to win most to complete the four majors," the American told reporters. "I think about it all the time.
"I have to put that out of my head, be patient and not think about results."
Mickelson, who won the last of his five major titles at the British Open in 2013, added: "I'm excited. I view this week as a great opportunity to complete something that would be historic in my mind."
The Californian came into the tournament under something of a cloud after agreeing to return almost $1m he made in profit from an insider trading tip.
"After a multiple-year investigation which led to nothing, no charges or anything, I think that kind of says enough for me," he said.
World No.1 Jason Day was also still to tee off but likes his chances at a course he says "almost feels like home", given his agent lives nearby.
"This is one tournament that is very stressful and I feel like I thrive under stress, and hopefully I can do that this year," said the Australian who won the U.S. PGA last year for his maiden major title.
The first round will resume on Saturday morning at Oakmont as organizers hope to get back on track to avoid a Monday finish.
Ironically, the U.S. Open is the only major to retain an 18-hole Monday playoff, so further lengthy delays could extend play into Tuesday.