Justin Thomas battled to stay among the frontrunners at the CJ Cup at Nine Bridges, as Luke List surged to the top of the leaderboard to lead by one from Lucas Glover and Scott Brown.
American Thomas, the 2017 FedEx Cup and US PGA Championship winner, was in sublime form in Thursday's opening road in Jeju, carding a nine-under-par 63.
But it was a trickier task for Thomas amid windier conditions during Friday's second round, and the world number four was three over par by the turn - although a classy putt with a wedge at the fifth displayed his qualities.
Thomas breathed life into his round by draining a 15-foot putt for birdie at the 10th, only to immediately hand that stroke straight back.
Back-to-back birdies briefly put Thomas back at the top of the pack, but a bogey at the last left him two off the pace in a tie for fourth.
At one stage, as many as six players were tied for the lead at six under, yet by the close of play it was List - who has never won a tournament on the PGA Tour - that led the way.
The American signed for a five-under 67 to leave him nine under for the tournament, as he hit five birdies in a brilliant blemish-free round.
Glover will start Saturday just one stroke adrift of his compatriot after a fine second round, which included a lengthy putt for birdie at the 11th, of 67 - seven birdies negated slightly by two dropped shots.
In a congested leaderboard, Brown followed an opening-round 66 with a battling two-under on Friday.
The American was two over for his round through 10 holes, but five birdies - including one at the last - and just one bogey through the final eight holes left him level with Glover.
Australian Cameron Smith is alongside Thomas in fourth, while his compatriot Marc Leishman and Patrick Reed are among four players tied for sixth.