The 31-year-old, playing on his home Royal Johannesburg and Kensington Golf Club, produced six birdies and a chip-in eagle at the ninth to join German rookie Maximilian Kieffer at the top of the leaderboard.
"Some days it feels good and everything kind of happens," 2008 Joburg Open champion Sterne told reporters.
"Golf is strange - you've got to hit the right kind of shots at the right time. I hit 17 greens today so I was never really in trouble at all," added the five-times European Tour winner.
Second-tier Challenge Tour graduate Kieffer, 22, like Sterne playing on the easier West Course, sprinkled his round with eight birdies.
South African Bryce Easton recorded a hole-in-one at the par-three 12th on the East Course on his way to taking third place on 65.
Britain's Ross McGowan was in the hunt going into the closing holes but he bogeyed the 16th and 17th to finish with a 67.
Norwegian Espen Kofstad also looked like he might grab a share of the lead before a triple-bogey seven on the 18th meant he ended up with a 66.
World number 18 Charl Schwartzel, the highest-ranked player in the field at the European Tour event, opened with a 68 on the East Course.
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