Pre-tournament favourite Justin Thomas remained in touch at the inaugural CJ Cup Nine Bridges, despite a two-over-par 74 in the second round.
The world number four — the highest ranked player in the field on Jeju Island — started the day with a three-shot lead after a superb opening round of 63.
But he was unable to hit those heights on Friday and finished two shots behind new leader Luke List.
The US PGA champion dropped three shots on the front nine and only marginally improved after the turn, mixing three birdies with two bogeys.
Thomas’ slip allowed List to hit the front in the battle for the €1.3m ($1.6m) first prize.
List shot a flawless round of 67 which featured five birdies to move to nine under in his bid for a first PGA Tour title.
Ireland’s Graeme McDowell shot his second consecutive 73, with a run of three bogeys from the 10th to 12th holes derailing his round.
Lucas Glover also enjoyed a five-under round to shoot up the leader board and sit one shot off the lead, alongside Scott Brown.
Marc Leishman and Patrick Reed, on six under, formed a dangerous chasing pack, with European Ryder Cup star Thomas Pieters one shot further back.