Just two strokes behind Grace was the ominous figure of Australian world number one Jason Day, who made an explosive start with four birdies in his first five holes on the way to a matching 66 that included an eagle at the par-five fourth.
On a gripping afternoon of blazing sunshine, five players held at least a share of the lead and there were frequent changes at the top as Oakmont lived up to its fearsome reputation for risk and reward.
The third round is scheduled to resume at 7 a.m. ET (1100 GMT) on Sunday before the regulation 72 holes are completed by the end of the day.
