Donaldson made three birdies in an outward 33 and has six holes to play. Bjorn needed only six holes to reach 3 under.
Boosted by his victory at the Phoenix Open, Donald's quick start at Sun City came off sweetly struck iron shots into the greens at his opening two holes. He also picked up back-to-back birdies at the end of his opening nine.
"Lovely start," he said. "It doesn't get much better than three-three around here. Birdie, eagle and all of a sudden you are three under par through two holes. I played solid for those first 11 holes and hit a couple of very nice shots in that time that I was able to take advantage of."
Defending champion Martin Kaymer sits fourth on 2 under par, having followed an early eagle with a birdie and a bogey.
Race to Dubai winner Henrik Stenson is among eight players — including Peter Uihlein of the United States, Sergio Garcia and Charl Schwartzel — tied fifth on 1 under.
Ernie Els was 3 over after seven in search of his fourth title at Sun City and U.S. Open champion Justin Rose was 4 over after eight. Only Australia's Peter Senior, with three double bogeys in his first six holes to crash to 8 over through 10, was worse than Rose.
Having initially been kept off the course for nearly three hours through an earlier thunderstorm, the players came off for the second time in late afternoon and will all have to come back at 7:30 Friday morning to complete their opening rounds.
Contributing: Associated Press
