Immelman, who was named the PGA TOUR's Rookie of the Year in 2006, established a new 54-hole record at this event with his three-day total of 20-under-par 196. Vijay Singh owned the old mark of 201, which he set in 1997.
Immelman is three clear of former three-time South African Airways Open winner, Ernie Els. The three-time major winner shot his second consecutive, 6-under 66 on Saturday and is alone in second at minus-17.
Overnight leader Patrik Sjoland only managed a 3-under 69 Saturday in benign conditions at Humewood Golf Club. The Swede is third at 15-under-par 201.
Defending champion Retief Goosen, who posted an 11 on the par-5 17th hole in Thursday's opening round, carded a 4-under 68 in round three. He is tied for fourth place with Andrew Raitt, who shot the same number, at 13-under-par 203.
Immelman, who won this title in 2003 and 2004, started the third round one shot behind Sjoland and collected his first birdie of the round at the par-4 fifth.
At the par-3 sixth, Immelman knocked his tee ball to 7 feet and converted the birdie putt. He added another birdie at the par-5 seventh despite an errant drive and was tied with Els atop the leaderboard at 14 under par.
Immelman, who turned 27 on Saturday, birdied the 10th and 11th holes and Els only managed a birdie at the seventh and a bogey at the par-5 11th. That swing gave Immelman a three-shot margin with the back nine to play.
Els got one back with a 7-foot birdie putt at the 13th. At the par-5 15th, Els hit a 5-wood for his second shot and landed the ball 8 feet from the hole. He sank the eagle try, but Immelman made birdie from 5 feet to keep a one-shot cushion.
Both players birdied the short, par-4 16th, but from there, Immelman took control.
At the par-5 17th, Els drove left and made par, while Immelman hammered his tee ball down the fairway. Immelman made birdie to move two ahead, then birdied the last to post his three-stroke edge with one round to go.
'Any time you can shoot a good round in Ernie's presence is a fantastic feeling,' Immelman said. 'My ball striking was not too good yesterday and I went on the range and had a long practice session - I smoothed a few things out.'
That is not to say Els is out of it.
'I know what I have to do in the last round and I'm feeling good at the moment,' said Els. 'I was very hard on myself in the past few weeks but this week I'm feeling good about my game again.'
Darren Fichardt fired a 65 on Saturday and is tied with Lee Westwood, who posted his third consecutive round of 4-under 68. The duo is knotted in sixth place at 12-under-par 204.
Charl Schwartzel (65), Ross Fisher (66) and Edoardo Molinari (65) share eighth place at minus-11.