The 36-year-old van Zyl was one of nine players parked on five under par halfway through his round before a triple-birdie burst around the turn propelled him into the lead.
He held onto that with a run of pars down the back nine before birdies at 16 and 18, where he hit a superb approach over the water to three feet, gave him the outright lead at 10 under.
Two shots further back was Germany's Maximilian Kieffer, who signed for a 65, with Bernd Wiesberger of Austria alone on seven under thanks to a 66.
French number one Victor Dubuisson (69) and former world number one Martin Kaymer of Germany (69) were both four shots off the lead, level with James Morrison of England (68).
Most of the fireworks early in a day when the temperatures again soared into the mid-30s came from Thai veteran Thongchai Jaidee.
Having just made the halfway cut by two strokes after rounds of 70 and 74, Thongchai needed a big early effort to get back in contention and that he did by birdieing the first six holes.
That shot him up the leaderboard to four under for the tournament through 14 holes, matching the overnight leading mark.
Thongchai, who has won six times on the European tour, came close to winning the French Open last year before Graeme McDowell triumphed.
McDowell, who was aiming to make it three French Open titles in a row this week, failed to make the cut on Friday as did former world number one Lee Westwood.