Dyson sank par putts of seven and 10 feet at the seventh and ninth, so turned in a fine 34.
He was still one in front, but it was now Larrazabal - third last year - who had become his closest challenger after a superb chip on the long 13th brought him his fourth birdie in six holes.
Campillo three-putted the eighth to be in third, with Bourdy and Ireland's Peter Lawrie tied for fourth three strokes behind.
Dyson almost spun his approach to the tenth into the hole and after tapping in for another birdie scrambled pars on the next two thanks to putts of 15 and eight feet.
He was seven under and went three clear when Larrazabal bogeyed the last for a 69.