Donald's fellow Englishman Danny Willett hauled himself level at the top of the leaderboard with the former world number one thanks to a superb seven-under 65 on Saturday.
However, Donald, playing directly behind Willett, kept his cool at the last to go 13-under overall and set the pace heading into the final day in Sun City, having handed the chasing pack a chance to close the gap with bogeys at 13 and 17.
Willett started the day five shots adrift of overnight leader Donald, but wasted little time in picking up four birdies on the front nine. Four more after the turn was offset slightly by a bogey at the 14th.
Having finished fourth at the Turkish Open last month and seventh at the Portugal Masters in October, the world number 84 is finishing 2014 in impressive form and his form on Saturday gives Donald plenty to ponder.
A third Englishman, Ross Fisher, is third, three shots behind Donald after replicating the 70 he shot 24 hours previously.
Fisher enjoys a five-stroke advantage of nearest rival Marcel Siem, with Louis Oosthuizen and Thongchai Jaidee next on four under.
Brendon Todd almost sunk a hole-in-one at the 16th, only to watch the ball trickle agonisingly close to the pin.