Already softened by two inches of overnight rain, Oak Hill's challenging East Course was expected to become even more receptive when the players returned to the par-70 layout.
Earlier, horns had sounded to halt the action as thunder rumbled overhead, and heavy rain then followed during the first interruption at the year's final major.
At that point, American Jim Furyk was the early leader in the clubhouse after firing a five-under-par 65.
Also at five under was Masters champion Adam Scott of Australia, who had reeled off five consecutive birdies from the par-five fourth and still had eight holes to complete.
Canadian journeyman David Hearn opened with a 66 to sit one stroke off the pace, level with Spaniard Miguel Angel Jimenez who had played 12 holes.
Tournament favourite Tiger Woods, hunting his first major victory since the 2008 U.S. Open, double-bogeyed his final hole, the par-four ninth, for a 71 to trail by six strokes.
(Reporting by Frank Pingue)
