Donald had picked off four more birdies in another blemish-free round to move to 14-under-par after 12 holes, four ahead of Danny Willett and Ross Fisher.
That trio are five shots ahead of fourth-placed Marcel Siem and Thongchai Jaidee, so a fourth English win at Sun City after the triumphs of Lee Westwood in 2010 and 2011 and Nick Faldo in 1994 seems inevitable.
Donald’s astute iron play and general nous around a course that demands plenty of strategy and sound judgement saw him gain shots at the par-three fourth and seventh holes, and the two par-fives around the turn.
The former world number one is in a great position to celebrate his birthday on Sunday with a victory that marks his return to the highest echelons of the world game.
Fisher was not able to close the gap on Donald, despite playing solid golf as he went two-under through 12 holes. The tall 34-year-old picked up birdies on the second, fifth and ninth holes, but found trouble on the par-four third when his drive landed in thick rough and he had to chip out sideways on his way to a bogey.
Willett was on fire on the front nine, turning in 32 after birdies on the second, fifth, sixth and ninth holes, and he kept the pressure on Donald with further birdies on 10 and 11. A good par-save from the greenside bunker on the par-three 12th kept him on track but a poor approach on the 13th saw him eventually drop a shot.
Donald found the middle of the green on the 195-metre par-three fourth hole and sank a lovely 20-foot putt for his first birdie of the day and Alexander Levy, playing in the final three-ball with Donald and Fisher, also birdied the hole, but found himself in trouble on the third and sixth holes, the Frenchman dropping shots on each to slip back to five-under-par. A double-bogey on the par-three seventh and another dropped shot on the 10th pushed him further back.
South African hope Charl Schwartzel suffered a disastrous double-bogey seven at the 14th, a wayward tee shot being compounded by a visit to the love-grass and the bunker in front of the green, and is back on two-under for the tournament.