Horne's nine-under-par total of 133 was one better than Richard Kaplan, who birdied the closing hole for a 69. Trevor Immelman, who fought back from a double bogey on the third to make 68, and Darren Fichardt, who had a 69, were one shot further back.
First round leader Grant Muller should have been on the same score, but somehow both he and his marker contrived to put down a par four on the sixth hole, when he had in fact made a three, so he ended up with a 72 instead of a 71.
Horne, a 29-year-old from Pietermaritzburg, said his lofty position on the leaderboard was 'unexpected. I haven't exactly had the greatest summer tour. I've pretty much played well the whole year, but just never got anything going,' he said.
Unlike Muller on Thursday, Horne was able to get away with missing a few fairways. 'I got pretty lucky with the lies,' he said, adding: 'My irons, particularly my short irons, have been very strong and I've been taking advantage of the birdie holes.'
Horne also managed to birdie two of the most difficult holes on the course, the par-four 15th, which is usually played as a par-five here and measures 476m this week, and the tough par-three seventh, which yielded only five birdies on Friday and saw 36 players in the 75-man field make bogey or worse, including two triple bogeys.
At one stage, Horne was in a seven-way tie for the lead at six under, but broke out of that with birdies at 13, 14 and 15. 'When I got to nine, I saw that I was seven under and there was a whole group on six under. I got very defensive from the ninth to the 12th - where he bogeyed to join that big group -but a 12-footer for birdie at the (par-five) 13th started things going again.'
Immelman put himself in front in the battle with Roger Wessels, who was one behind him after a 67 and Bradford Vaughan, who fell back to three under after a 74, to finish second on the Sunshine Tour order of merit.
However, he said he was trying not to think too much about it. 'I'm here to win the golf tournament. If I win, they can't beat me. I'm trying to keep my mind off it,' he said.
Referring to the big invites, including one to this year's British Open, that second place would bring, he said: 'I'd love to get into those, but what I've got on my side is that I'm so young, so I'm eventually going to play in those tournaments. I can only do my best this week and if that's not good enough, so be it,' said Immelman, who has sewn up the Rookie of the Year title.