Korea's Byeong Hun An delighted the home crowd by carding an eight-under-par 64 to lead the CJ Cup at Nine Bridges on Jeju Island.
An fired eight birdies in a bogey-free round to lead Chile's Joaquin Niemann by a shot, with former world number one Jason Day a shot further back following a 66.
Graeme McDowell is inside the top 10 on four under. The Portrush man had four birdies in a flawless round of 68.
Defending champion Brooks Koepka is five shots off the pace after a 69 matched by the English pair of Ian Poulter and Tyrrell Hatton and Norway's Viktor Hovland, whose 18th consecutive round in the 60s established a new PGA Tour record.
Koepka's erratic round included four birdies, three bogeys and an eagle on the par-five 18th and the world number one said: "Just needed to clean up a few things, missed a couple of short putts, a little sloppy there, but other than that I thought I hit the ball good.
"The wind was just so light today it felt like it was switching quite a bit and it made a difference on a couple of holes.
"Honestly I think this golf course is a little bit easier if it blows. If it blows a little bit the wind is a little bit more consistent."
Day, who is hoping International Team captain Ernie Els will give him a wild card for December's Presidents Cup at Royal Melbourne, boosted his chances by carding six birdies in a bogey-free 66.
"It is certainly disappointing not to make the team on my own merits, but it is a reminder that nothing is given to you in this game," the Australian said.
"I certainly want to be on the team and believe I can be an asset to the Internationals. Being in my home country certainly gives me an edge and I've had some success on the sandbelt (courses) and Royal Melbourne in the past."
