Clark, who missed almost all of last year with a wrist injury that required surgery, had six birdies and two bogeys on his round under windy conditions at Durban Country Club.
'I've played here in the wind many times before and I enjoy it,' said Clark. 'I will be happy if it does blow as I am swinging really nicely. If I can keep hitting the greens and leaving myself with easy par putts, then hopefully the birdies will come.'
The 26-year-old began the day three shots behind second-round leader James Kingston and quickly made his move on the front nine. After making birdie and bogey on the third and fourth, Clark birdied five of the next six holes to jump to 13-under.
He bogeyed the 11th and played even par the rest of the way to finish at 12-under 204.
Clark, who earned his PGA Tour card last year after finishing third on the Buy.Com money list in 2000, will play the 2002 campaign under a medical exemption. He will make the trip to the U.S. for the Pebble Beach National Pro-Am at the end of January.
Goosen was four shots off the lead until an eagle on the par-5 eighth. The U.S. Open winner hit his approach on the 501-yard par-5 just off the fringe, setting up a chip-in to pick up two strokes.
'I am playing OK but not great,' said Goosen, who won the South African Open at Randpark Golf Club in 1995. 'I'm struggling to read the greens, but hopefully tomorrow I can make a couple of putts to get me going. It's very tight at the top so it's going to be an interesting day.'
He added two more birdies and a bogey to finish the round at 10-under 206.
Lomas, the leading Englishman after the third round despite bogeying the first two holes, rolled in a three-foot putt for birdie on the 18th to join Goosen two shots off the lead.
Kingston, who began the round with a two-shot lead over Lomas and Andrew Butterfield, climbed to 13-under with two birdies over the first three holes before running into trouble on the back nine.
The South African struggled to a 74 with five bogeys over the final eight holes to go along with a lone birdie on the 11th. He is joined by fellow countrymen Roger Wessels (68) and Jean Hugo (68), England's Steve Webster (69) and Alastair Forsyth (70) of Scotland at 9-under 207.
Jamie Donaldson of Wales had the low round of the day with a 6-under 66. He finished tied for ninth alongside South Africa's Martin Maritz (72).
England's Simon Dyson (69) was joined by fellow countryman Butterfield (74) at 7-under 209. They were followed by a larger group at 6-under that included Ernie Els, who had five birdies and one bogey for a round of 68.
Full-Field Scores from the Bell's South African Open