Henrik Nystrom is alone in third place at 4-under-par 69 on the par-73 Fancourt Golf Club, which hosted the 2003 Presidents Cup. Mattias Eliasson and Charl Schwartzel share fourth place at minus-3.
Two-time U.S. Open champion Retief Goosen, who won this title in 1995, managed an even-par 73 on Thursday and is part of a large group tied for 17th place.
Ernie Els, a three-time winner of this event, struggled to a 3-over-par 76 and is tied for 52nd place. Els, who returned to competitive golf two weeks ago, captured last week's dunhill championship.
Clark began on the back nine Thursday and parred his first six holes. He collected his first birdie at the par-5 16th. Clark, who also won in 2002, took advantage of the next par-5, the 18th, when he ran home a 20-footer for eagle.
He parred No. 1, then ran off back-to-back birdies at the second and third. Clark parred four in row after the birdies, then got to 6 under par with a birdie at the par-3 eighth.
Clark hit his worst shot of the round at the par-5 ninth. He could not recover from a poor drive and bogeyed the hole, falling back into a tie for the lead.
'I merely picked a target today and tried to hit it there,' said Clark, who represented the International side at the Presidents Cup two years ago. 'It's all about making the right decisions around this golf course. I tried to be smart and hit the middle of the greens.'
Clark has logged quite a few miles in recent weeks. He played in the Nedbank Challenge in South Africa two weeks ago, then flew to California for the Target World Challenge. Now it's back to his home country and a course where he's comfortable.
The course suits me because there is no particular way to play it,' said Clark. 'I just kept playing away from the flag and kept it safe. I certainly did not go out there and attack the golf course.'
Fisher, who played most of the 2005 season on the Challenge Tour, also started on the 10th tee Thursday. He was awesome on his opening nine with five birdies to make the turn at 32.
It was a much different second nine for Fisher on Thursday. He bogeyed the first hole, then got back to minus-5 with a birdie at the 549-yard, par-5 fifth.
He fell one behind Clark with a bogey at the par-3 eighth, but a birdie at the last gave him his piece of the first-round lead.
James Kingston, Jaco Van Zyl and Francesco Molinari are knotted in sixth place at 2-under-par 71.
Warrick Druian, Oliver Whiteley, Anton Haig, David Lynn, John Bele, Michael Hoey, Andrew McLardy and Gregory Havret are knotted in ninth place at 1-under-par 72.