Jeju Island - Scott Piercy led a dominant American leaderboard at the PGA Tour's CJ Cup at Nine Bridges on Friday with a 7-under 65 to take a one-stroke lead after two rounds.
Piercy had a 36-hole total of 9-under 135, one ahead of Brooks Koepka, who also shot 65 and is playing in his first tournament since being voted PGA Tour player of the year.
Fellow American Chez Reavie, who led by one stroke after the first round, was in third, three strokes behind Piercy.
Sweden's Alex Noren (65) and Ian Poulter (69) of England were tied for fourth, four behind.
Five Americans - Brian Harman with a round-of-the-day 64, Ryan Armour and Gary Woodland (67s) and Jamie Lovemark and Pat Perez (68s) - were tied for sixth, five strokes behind Piercy.
Defending champion Justin Thomas shot 70 and was eight shots off the lead at 1-under.
135 - Scott Piercy 70-65
136 - Brooks Koepka 71-65
138 - Chez Reavie 68-70
139 - Alex Noren (SWE) 74-65, Ian Poulter (ENG) 70-69
140 - Ryan Armour 73-67, Jamie Lovemark 72-68, Pat Perez 72-68, Gary Woodland 73-67, Brian Harman 76-64