The 28-year-old - who would emulate compatriot Bobby Locke's victory in 1938 - shot a scintillating six-under-par 66 to sit on 14-under for the European Tour event and tantalisingly in sight of his first win on the EPGA Tour.
Van Rooyen's closest pursuers are first round leader Ryan Fox of New Zealand - the son of All Black flyhalf great Grant Fox was joint fourth last year - and Sweden's Joakim Lagergren, who has returned to form after missing several cuts since landing his maiden tour win in the Sicilian Open in May.
"The mindset was just keep going, I guess," said van Rooyen.
"I wasn't very much aware of my score or the situation.
"You sometimes get in a groove as a golfer where things just happen and you hit great shots and great putts and that's what happened."
England's Danny Willett is another player who has shown signs of a return to his old self, injuries and a loss of confidence on the course - this season alone he has missed nine cuts in 12 tournaments - seeing the 2016 Masters champion slump to below 400 in the world rankings.
However, the 30-year-old is in contention for at least a welcome large pay cheque and even victory if van Rooyen implodes after shooting a three-under par 69 and sits five off the South African.
Willett, who rattled in four successive birdies, says he is not intimidated by the gap between himself and van Rooyen.
"It was nice to finish five shots back and there's still a lot to play for," said Willett.
"When you've got such a big lead it's a tricky place to be."