A nine-under-par round of 63 sent Justin Thomas to the top of the leaderboard at The CJ Cup in South Korea.
Thomas, the champion at Nine Bridges two years ago, had nine birdies, including four in the first four holes, and no dropped shots to move to 13 under after two rounds.
That left the American two shots ahead of first-round leader An Byeong-hun, who followed up a 64 with a 69.
Graeme McDowell fell away from the top of the leaderboard as he hit 71 to move to five under par.
Korean-born New Zealander Danny Lee is tie for second with An after a 66. It would have been one better but for a bogey at the par-five 18th.
American Jordan Spieth and Argentina's Emiliano Grillo are two shots further back in a tie for fourth. Spieth was one of the big movers of the day thanks to a 65.
England's Tyrell Hatton is in a tie for ninth place on seven under after shooting a 68 while a 73 dropped Australia's Jason Day from third to a tie for 18th.