"The mindset was just keep going," Johannesburg-born Van Rooyen said. "You sometimes get in a groove as a golfer where things just happen and you hit great shots and great putts and that's what happened."
Van Rooyen followed up with nine straight pars to finish at 14-under overall, four shots ahead of Kiwi Ryan Fox and Sweden's Joakim Lagergren.
Defending champion Jon Rahm surged into the top 10 after a 67 left him in a group on six-under alongside fellow Spaniard Jorge Campillo, England's Andy Sullivan, American Peter Uihlein and South African's Zander Lombard.
Rahm sank four birdies in his first five holes, and after dropping a shot at the seventh, roared back with further gains on the next two holes.
Tournament host Rory McIlroy felt he was missing an extra gear for a third consecutive day on the greens as he signed off on a 72, with four birdies and as many bogeys.
"It was pretty similar to the last couple of days", said McIlroy, who finished the day 13 shots off the pace.
"I hit the ball pretty well, a couple of loose shots here and there, and holed a couple of putts out there I guess.
"But with that little bit of rain coming in the middle of the round, I felt like the greens must have slowed up a little bit. I struggled to get the ball to the hole the last few holes."
(Reporting by Hardik Vyas in Bengaluru; Editing by Christian Radnedge)
