Rock shot 4-under 67 in his second round and is one stroke ahead of Jbe Kruger and Braden Grace, who each shot 5-under 66. Michiel Bothma and George Coetzee are two strokes behind Rock. Bothma also shot 66 for the second round.
But Coetzee is among the majority of players who are yet to finish 36 holes -- Thursday's co-leader Jamie Elson didn't even begin his second round -- because of weather.
Lightning and thunderstorms marked Thursday in Johannesburg, stopping play for about four hours and leaving Rock with two first-round holes to finish. He birdied both Friday.
Another series of storms pushed play back Friday. Coetzee got in six second-round holes and is the only player other than Elson in the first eight not to have completed 36 holes.
Rock, who opened his tournament an eagle, posted a 3 at the par-5, 539-yard second hole Friday. He had a bogey at No. 8 but got that back with a birdie at the ninth. After another bogey at 12, he moved into the lead with three consecutive birdies over 13-15.