The 32-year-old American started on the back nine on Thursday and answered a bogey at the par-three second hole with back-to-back birdies at three and four, the latter from almost seven metres, and sank a three-foot eagle putt at the par-five fifth to reach six-under par.
Clark, the 2023 US Open champion who says he’s seeking redemption after smashing a locker at Oakmont over missing the cut at last year’s championship, parred six after a 61-foot chip to within inches of the hole then parred the seventh before play was halted.
“It was a nice start for sure,” he said. “It was a tough day. It was tough for everybody, I think. Biggest thing for me, I stayed patient. Didn’t have my best stuff, but I made a lot of putts and saved myself to keep the momentum going.
“Happy with where I’m at for sure.”
Not since 1933 has a player led the US Open by four strokes after 18 holes, which Clark can achieve when he plays the eighth and ninth holes when the round resumes early on Friday.
The lowest 18-hole score for a US Open at Shinnecock is four-under 66.
Sharing second on two-under par 68 in the clubhouse were amateur Ryder Cowan and fellow Americans Sam Stevens and Max McGreevy.
Four major winners were still on the course at two-under – Spain’s Jon Rahm after 13 holes, Americans Dustin Johnson and Gary Woodland after 15 holes and England’s Matt Fitzpatrick after 16 holes.
Johnson, a back-nine starter, had reeled off four consecutive birdies through the fourth hole but fell back with a three-putt double bogey at the sixth.
Among those five strokes back sharing ninth in the clubhouse on 69 were six-time major winner Rory McIlroy and Sweden’s Ludvig Aberg.
Fog struck only 30 minutes after the round began and stopped play for two hours, a delay that saw 50 players stranded on the course at sunset.
Gusting winds blew away the fog but combined with thick rough and greens slightly softened by watering to test the world’s top golfers.
Rahm made one of the day’s best shots, a stunning 61-foot birdie putt at the par-three 17th, to reach two-under.
McIlroy, another back-nine starter, shot 69 after closing with back-to-back bogeys after an 11-foot eagle putt on the fifth.
“Overall, a really good day,” McIlroy said. “Obviously it stings a little bit to finish the way I did.”
A field of 156 is chasing a record top prize of US$4.5 million from a record US$22.5 million purse. (AFP)