Defending champion Stephen Gallacher began the day with a two-shot lead but bogeyed the first two holes after wayward drives and dropped further shots on the sixth and eighth.
That meant playing partner Rory McIlroy's run of six straight pars was enough to inherit the lead, but the former world number one then promptly found the water with his tee shot on the par-three seventh.
A brilliant third shot from the drop zone to three feet meant McIlroy escaped with a bogey, but meant American Brooks Koepka took over at the top of the crowded leaderboard by chipping in for a birdie from the back of the eighth green.
At 14 under, Koepka - who won three times on the Challenge Tour last season to gain instant promotion to the European Tour - led by one from McIlroy, Steve Webster and Finland's Mikko Ilonen, who had set the clubhouse target after a 64 which featured two eagles, five birdies and one bogey.
Gallacher was two shots off the pace on 12 under and in need of a similar finish that he produced in his third round of 63, where he covered the back nine in just 28 shots.