Oakmont - Dustin Johnson held the US Open clubhouse lead Saturday as surprise first-round leader Andrew Landry set off with some half the field in the second round of the weather-hit championship at Oakmont.
World number six Johnson had a two-round total of four-under par 136 after playing both his first and second rounds on Friday.
His nearest rivals in the clubhouse were Spain's Sergio Garcia and American Scott Piercy who both carded second-round 70s on the formidable Pennsylvania course to stand at two-under for the tournament.
American Jim Furyk wrapped up a two-under 68 for 139, joining England's Andy Sullivan and American Daniel Summerhays in the clubhouse on one-under.
Two-time Masters champion Bubba Watson, New Zealand's Danny Lee and American Kevin Streelman were also at one-under as they headed into the second round.
After weather delays on Thursday, only half the field started the second round on Friday, with the rest teeing off on Saturday morning.
Landry, ranked 624th in the world, opened the second round with back-to-back pars to keep pace with Johnson.
He and others were facing a marathon day at Oakmont, where warm, sunny weather was already firming up the treacherous greens.
Defending champion Jordan Spieth had some ground to make up after posting a two-over first round.
World number three Rory McIlroy was in even tighter straits, battling to make the cut after a seven-over first round.
World number one Jason Day and Phil Mickelson, meanwhile were playing a waiting game.
Day looked safe for the halfway cut after a second-round 69 put him at five-over.
But Mickelson, a six-time runner-up in the US Open - the only major to elude him - completed his second-round 73 Saturday morning for a seven-over total of 147.