A storming outward 31, including five birdies, gave Donaldson a three-stroke halfway lead at the Gary Player Country Club on a sombre day following the death of former South Africa president Nelson Mandela.
Friday's early start was preceded by a minute's silence in honour of the 95-year-old statesman as the elite 30-man field resumed their weather-interrupted first rounds from Thursday.
Sergio Garcia finished as first-round leader after a blistering morning start as he posted a 66 but Donaldson was already out on the course again, picking up three birdies in his first four holes.
On the par-five 10th, he carded a bogey six after picking up an unusual penalty.
"It was mad, I just pushed my tee shot a little bit into the green-side trap and tried to move it about 100 yards down the fairway but it clipped the lip and as the ball sort of came back I hit it again with my shaft, over my shoulder, back into the bunker," the Welshman told reporters.
The referee's ruling was a one-shot penalty but Donaldson recovered to sink two more birdies to finish six-under for the round.
His 66 was only bettered by American Ryan Moore, who made seven birdies in a second-round 65 that took him eight-under and into joint second place with Sweden's Henrik Stenson.
Birdies on the last hole of the day for both defending champion Martin Kaymer of Germany and Dane Thomas Bjorn took them to seven under par.
- Sports & Recreation
- Gary Player Country Club