The victory is the eighth of his European Tour career and first since the Qatar Masters early previous year; an emotional win in the tournament styled as "Africa's major".
"I've played with some of the best in the world this week and proved I can compete", he said.
"I've got to see if I can step up to the challenge".
South Africa's Haydn Porteous posted a five-under 67 to be at three-under, while Race to Dubai leader Tommy Fleetwood hit back after a hard first two days to also post a 67.
The Scot had shot rounds of 66 and 57 on a shortened and rain-affected Durban course to win in a play-off in what was the first of two Nelson Mandela Championships. "I think to be in with a chance of winning the Race to Dubai, it's pretty special", said the player from Southport.
As his nearest challenger, Dubuisson will also like his chances.
Cape Town - England's Jordan Smith has withdrawn from the Nedbank Golf Challenge, hosted by Gary Player, following the second round on Friday.
"It's one of my favourite tournaments".
"Hopefully you've always got something to play for towards the end of the year, whether it's top 30, gets you in The Open or Top-10, Race to Dubai, and it's a new experience trying to win it".
It's an approach that also suits Grace.
He said: "I was much better on the greens today than I had been the first couple days, so that was nice".
"The key is to get a good start, get that front nine under control, like I did today, and then push on the back".
He recovered well with a birdie at 9, but once more a bogey on 10 halted his progress. If he can change that and start well on Sunday, South Africans may be celebrating a home victor by the evening.
Having once again come out on top of another star-studded field, Grace will hope for more good things at the tour finale in Dubai later this week, and in the Majors next year.
He made five birdies in another steady round of golf which has left him in pole position to win "Africa's Major" on Sunday.
Jamieson will be joined in that event by 29th-ranked Richie Ramsay and David Drysdale (54th), with Marc Warren agonisingly missing out by one spot after he slipped to 63rd following a disappointing finish in South Africa.