Van Zyl began the second round with a three shot advantage, but saw his lead cut to two strokes by a charging Richard Bland and Chris Wood from England.
Blanch reeled in seven birdies on his back nine for a 65 and Wood joined him in second with a 66, but the 36-year-old Dainfern golfer hung tough to finish at 14-under-par 130.
Van Zyl opened with a 61 that would have set a new course record had preferred lies not been in operation, but said his opening gambit didn’t figure into his second round.
“I’ve been in this game long enough to know you can shoot a low number today and an 80 in the next round,” said Van Zyl.
“I consciously didn’t think about it and took things a hole at a time. I got off to a nice start with four birdies on the front nine and I was seven shots clear at the turn. Unfortunately a couple of loose shots on the back nine brought the pack to me.
“I still hit it great and I’m really pleased to be in this position in this calibre event.”
Van Zyl’s demeanour has been likened to that of countryman and two-time US Open champion Retief Goosen, who is known as ‘The Iceman”.
He barely blinked when two shots on the back nine died a watery death.
“You always going to have a bad hole somewhere, and mine came at four and eight,” he said. “The wind blew harder in the first round, so you could commit to your shots.
“Today the wind was softer, but swirled a lot and changed direction. It makes it more difficult to judge and you start doubting yourself.
“I had 260 yards to clear the water at four and get on the green in two. I thought I hit a beauty, but the wind caught the ball and it came up short and caught the water. At eighth I hit an eight iron into the par three in the first round.
“With the wind into us, I thought a seven-iron would be perfect, because you don’t want to hit it long. I wasn’t totally committed, though, I pushed it. It finished short and rolled in the water.
“Other than those two holes, the round was really good, but the putts didn’t drops as many times as the first round. I didn’t make as many putts today, but I’m striking the ball well. I’ll do a bit of work on the short game this afternoon to get it sharp for Saturday.”
Victor Dubuisson from France, the champion in 2013, fired the low round of the day – a 64 to vault to joint fourth at 11 under with Fabrizio Zanotti from Paraguay, who carded a 66.
Former world number one Rory McIlroy also joined the queue trying to chase down Van Zyl. The Major champion carded a 67 to tie Frenchman Alexander Levy (65) at 10 under.
South Africa’s Thomas Aiken shot a 72 to finish at four-under, while Charl Schwartzel and Trevor Fisher Junior are at two under after respective rounds of 71 and 72.