Overnight leader Titch Moore's three-shot lead over Immelman had melted away by the fifth when his third successive bogey was met by Immelman's birdie four.
Moore promptly dropped at the sixth to give Immelman an outright lead he was never to surrender despite the building pressure from the Big Easy.
Although Els picked up only one shot on the outward nine his birdie, birdie, birdie from the turn left Immelman feeling the hot breath of the World No. 2 on his collar. Immelman's four shot lead at the turn had been slashed to one shot.
"I'm quite proud of the way I stood up to the pressure. I thought I'd be more nervous," said Immelman. "I just tried to stay focused on my own game and not worry about what anyone else was doing. "I played great but you have to say that it must have been a bit of an anti-climax for Ernie to come here after the great week he had at the Nedbank Challenge."
Els started with a birdie three at the first but gave that up at the par-five fifth as his up and down week continued. A drop followed two more birdies at the ninth before his run of three birdies from the tenth. All that good work was undone with drops at 14 and 15 to ease the pressure on Immelman.
The win followed Immelman's first pro victory in the Kenya Open on the Challenger Tour and completed a remarkable comeback from illness and loss of form.
"A year ago I was lying in a hospital bed with meningitis wondering if I'd ever be able to go out and there and shoot 80 again," he said. "My brother Mark came to me at the time and told me as a brother and as a golf coach what he thought I was doing wrong and what I needed to do with my swing. We set targets for myself and for him and win twice in the year is a good year."
Immelman produced a bogey-free round, hitting a tree and taking a lucky bounce back onto the fairway at the sixth and then sinking a clutch ten-footer at the par-three 13th to keep his round together.
Overnight leader Titch Moore had a horror round, birdieing the second but then having those four successive birdies as his accuracy from the tee deserted him.
He dropped another shot at the 13th but came back in 37 for a three-over par 75 to collect R184 000 for tieing second with Els to collect the biggest pay cheque of his career.
Only seven players ended with sub-par rounds as the south-easter continued to ask questions. Despite a lessening in the force of the wind there were only 17 sub-par rounds on the final day.