Martin fired ten birdies and a solitary bogey at Luxehills International Country Club to reach 20 under par, but on the course Colsaerts was 22 under with four to play.
The 28 year old - who was the longest driver on Tour last season - led by one overnight but showed great composure to add four birdies without a dropped shot over his first 14 holes.
Swede Christian Nilsson fired a 64 to finish the week 19 under, while New Zealander Danny Lee was on the same mark with two to play.
Nilsson and Martin were playing with England’s Danny Willett – who amazingly came back with the worst score of the three despite a 65, the trio’s combined effort coming in at 24 under par.
“Fantastic, great day - I’ve never seen so many birdies in one group ever,” said Martin, who was the first amateur to win on The European Tour.
“I think it must be some sort of record! It was good fun. I put a good score in there. I would like for Nicolas to win but I’m happy.”
Ireland's Peter Lawrie still had an outside shot at the title but from four shots back with five to play the former Open de España winner was going to need something special over the closing holes.