Luke Donald was joint leader of the US Open on Friday after the second round got underway three hours later than scheduled.
The late start was due to the thunderstorms that slashed over four hours of play from Thursday's first round and left half the 156-strong field stranded out on the course when darkness fell.
Phil Mickelson stayed atop the leaderboard at three-under 67 as the first round was completed Friday morning and it was England's former world number one Donald who made the first move as the second round started.
He had birdies at the 12th and 13th -- his second and third holes -- to get to four under, but a poor bunker shot at the 15th dropped him back into a tie at the top with Mickelson.
Australia's John Senden moved up to two-under with birdies at the fifth and sixth and he was level with compatriot Mathew Goggin, who completed a 68 earlier in the morning.
Scotland's Russell Knox and Nicolas Colsaerts of Belgium, on 69, were the only two other players under par at tough Merion Golf Club after the first round.
Tiger Woods stayed at three over after a birdie at 13 and a bogey at 14 as he began his second round, while playing partner Rory McIlroy got to two under after an early birdie.
Because of the rain delays, the second round would not be completed before Saturday morning at the earliest.