The 2010 Players Championship winner got off to a wobbly start when he bogeyed the third, but he reconsolidated over the next five holes and turned at one-over.
On the back stretch of Royal Montreal Golf Club he turned the day around with a dazzling run of three birdies of the 10th to the 12th. That secured a five-under total for the Durbanite, who added a final birdie at the 17th to reach six-under for the week.
Jim Furyk shot an impeccable 63 on Friday to join Tim Petrovic in a share of the lead on 10-under, but Canadian hope Graeme DeLaet loomed on eight-under.
“I really feel like I'm hitting the ball well,” said Furyk. “I'm rolling some good putts and really it's a matter of patience. I've had a lot of success here, so I've got some good feelings and really a lot of support here. But we're only at the halfway point. Tomorrow really is still jockeying for position."
Graeme McDowell held a share of fifth place at the halfway mark, while Matt Kuchar joined Clark in an eight-way tie for ninth place.