The American started the day in a four-way share of the lead, but excellent putting saw Young move to 13-under-par overall, despite a bogey on his final hole after hitting the water with his second shot.
"I putted fantastic," Young said. "I made a couple of long ones yesterday and then made a few more today that had no right going in."
Andy Sullivan was the only one of the leading quartet to challenge Young, holing five birdies on the front nine to card a five-under-par 67 and shares second place with Poland's Adrian Meronk, three shots ahead of their closest rivals.
Nicolai Hojgaard hit an eagle at the 17th to join his twin brother Rasmus and Scotland's Richie Ramsay on seven-under-par overall.
Masahiro Kawamura looked on course to threaten Ernie Els's course record of 61, set in 1994, with five birdies and an eagle on his first six holes, and was 10-under after 12, but the Japanese player lost momentum to card a nine-under 63.
England's Luke Donald hit a hole-in-one on the par three fourth hole as he shot a second consecutive 71.
"My eyesight is not that good these days," Donald said. "I couldn't see it but I saw some people behind the green jumping up and down."
