Richard Sterne was one shot behind on home soil, followed by 2005 champ Charl Schwartzel, among five players two strokes off the lead in the European Tour event at Leopard Creek.
Lee Westwood was three behind, and Rory Sabbatini five off the pace.
Mattiace, who shared the first-round lead with Swedens Michael Jonzon, added a 4-under 68 to his opening 65 with a clutch par save on the par-five 18th hole. After hitting his drive into the creek beside the hole, he took a penalty drop and made par from nine feet to cap a bogey-free round.
That was a great save, he said. But Im in a good spot for the weekend and Im very happy about it.
Rock birdied five of the last six holes to shoot 67 and join the leaders.
It was a decent round of golf, said Rock, seeking his first win on the European Tour.
Henningsson, playing in his first tour event this year, also birdied the 18th to cap the days best score, a 64.