Ardmore - The weather-delayed first round of the 113th U.S. Open resumed at 12:10 p.m. local (18:10 SA time) at Merion Golf Club with afternoon tee times delayed by three hours 45 minutes.
Less than two hours of play had been possible earlier in the day after the year's second major began in overcast but dry conditions before the siren sounded at 8:36 a.m. to halt the action due to the threat of lightning.
Players and spectators were advised to seek cover and the already rain-softened layout was saturated by a heavy downpour under darkening skies as thunder rumbled and lightning sparked.
Once the rain had abated, players were given the all-clear to head to the practice range to prepare for a resumption of play, though severe thunderstorms and high winds have been forecast for later in the day.
Englishman Ian Poulter was the early leader at three under par, having birdied his first three holes after teeing off at the par-four 11th.
Long-hitting Belgian Nicolas Colsaerts, South Africans Charl Schwartzel and Tim Clark and American Charley Hoffman were knotted at two under. Colsaerts had competed seven holes, Schwartzel and Clark were through four and Hoffman two.
English world number five Justin Rose had lined up a slick 15-foot downhill birdie putt at the par-four 15th shortly before play was suspended.
After barely touching the ball, he watched as his birdie attempt rolled two feet past the hole and then horseshoed out with his par putt to slip to one over for the round before the siren sounded to halt play.
Phil Mickelson, runner-up at the U.S. Open a record five times, was at level par after five holes, having offset a bogey on his opening hole, the 11th, with a birdie at the 13th.
The American left-hander had returned to Merion in the early hours of the morning on an overnight flight from San Diego, where he had attended his oldest daughter's eighth grade graduation ceremony.
Tournament favourite and world number one Tiger Woods was among the late starters, scheduled to tee off with second-ranked Rory McIlroy and Masters champion Adam Scott in the most eye-catching trio of the day.
However, Woods and company may not even tee off on Thursday with severe thunderstorms expected to hit the Philadelphia area in the afternoon.
As much as three inches (7.6 cms) of rain has been forecast for the day, and Merion Golf Club's iconic par-70 East Course has already been saturated with more than six inches since Friday.
Though the layout drains extremely well, the biggest concern for organisers is the green and greenside bunker at the 11th hole, the lowest point on the course.
Extra care has been taken over the location of pin positions on greens more susceptible to puddles, while the safety of players and spectators has been paramount in contingency plans.