The South African, two behind the American duo overnight, joined Matt Kuchar in a tie for third on eight under with his early gain.
Another American, Jim Furyk, was making the biggest move of the early starters.
The Ryder Cup stalwart eagled the par five second and with five further birdies and a solitary bogey in his first 13 holes the former US Open Champion was seven under with five to play.
Four other Americans - Bo Van Pelt, Nick Watney, Justin Leonard and Hunter Mahan - were also seven under during the early stages at Firestone, as was Swede Peter Hanson, who parred the first two holes.
Meanwhile Phil Mickelson’s wait to become World Number One was set to last at least another week.
The Masters Tournament winner needed a top four finish to end Tiger Woods 270 week stay at the top but a six over par outward 41 saw him slide from a tie for tenth down to 39th.