Jeju - Justin Thomas shot a stunning nine-under 63 on Friday on Jeju Island to take control of the CJ Cup from home favourite An Byeong-hun as world No 1 Brooks Koepka's title defence evaporated with a 75.
Overnight leader An was reeled in by the relentless Thomas who started four shots behind but finished two in front after setting the South Korea event's 36-hole tournament record on 13-under 131.
"That was, I would say, overall one of the better rounds I've played in a while," Thomas declared after equalling his own best round at the $9.75 million PGA Tour event.
"I just kind of kept the ball in play off the tee, hit some good irons. I think I hit every green and made a couple putts early," said Thomas, who also shot 63 in the opening round two years ago on the way to victory.
"And then the back nine, I got a few close to the holes where those birdies came easier. So it just was a very easy 63, if you can somehow say that."
South Korea's An, who shot 69, shares second on 11-under going into the weekend with Seoul-born New Zealander Danny Lee, whose 66 could have been better but for a bogey at 18.
Koepka, the winner at the Nine Bridges course a year ago, had a day to forget with five bogeys in his three-over par round and ended the day 13 shots off the lead.
The best player in the world wasn't even the best player in his family - little brother Chase Koepka shot a level-par 72 to go into the weekend at two-under par, two shots better than his famous sibling.
A rejuvenated Jordan Spieth rolled home five birdies in his last seven holes in a putting masterclass to card a 65 and move into contention.
The three-time major winner and former world number one shares fourth place on nine-under par with Emiliano Grillo of Argentina, who carded a 66.
The 26-year-old Texan has not won a tournament for two years but felt he was close to his majestic best on Friday.
"There's still some work to do for sure, I'd call it 'B' ball control but I was 'A-plus' putting," Spieth told AFP.
"Just got to go out tomorrow and hit as many greens as possible and keep rolling it the way I've been rolling it."
Thomas, the inaugural CJ Cup winner in 2017, blasted out of the blocks on a blustery morning on the side of Mount Halla, Korea's highest mountain that dominates Jeju Island.
He birdied each of the first four holes to get to eight-under par and another on the eighth put him within striking distance of the Korean.
An's round got going around the turn with four birdies from his eighth to 11th holes.
"I guess my putting was pretty good. I didn't miss any short putts and a couple of long ones dropped, that definitely helped," said An.
Thomas started the back nine with his sixth birdie and another on the par-five 12th drew him level.
A three at the par-four 14th gave him the outright lead and a two-putt birdie at the par-five 18th provided a two-shot cushion going into the weekend.
Scores on Friday after the second round of the PGA Tour's CJ Cup at The Club at Nine Bridges Golf Club, Jeju Island, South Korea (par 72):
131 - Justin Thomas (USA) 68-63
133 - An Byeong-hun (KOR) 64-69, Danny Lee (NZL) 67-66
135 - Emiliano Grillo (ARG) 69-66, Jordan Spieth (USA) 70-65
136 - Ryan Moore (USA) 69-67, Cameron Smith (AUS) 67-69, Lee Kyung-hoon (KOR) 69-67
137 - Kim Si-Woo (KOR) 69-68, Charles Howell III (USA) 67-70, Tyrrell Hatton (ENG) 69-68
138 - Wyndham Clark (USA) 71-67, Kevin Streelman (USA) 69-69, Kiradech Aphibarnrat (THA) 69-69, Hwang Jung-gon (KOR) 67-71, Joaquin Niemann (CHI) 65-73, Viktor Hovland (NOR) 69-69
139 - Hideki Matsuyama (JPN) 69-70, Jason Day (AUS) 66-73, Graeme McDowell (NIR) 68-71, Charley Hoffman (USA) 67-72, Ryan Palmer (USA) 70-69
140 - Kevin Na (USA) 72-68, Andrew Putnam (USA) 70-70, Rory Sabbatini (SVK) 71-69, Lee Soo-min (KOR) 68-72, Dylan Frittelli (RSA) 70-70
141 - Im Sung-jae (KOR) 68-73, Marc Leishman (AUS) 72-69, Ian Poulter (ENG) 69-72, Tommy Fleetwood (ENG) 71-70, Nate Lashley (USA) 72-69
142 - Phil Mickelson (USA) 70-72, Pat Perez (USA) 73-69, Harold Varner III (USA) 71-71, Gary Woodland (USA) 71-71, Chase Koepka (USA) 70-72, Kevin Tway (USA) 70-72, Collin Morikawa (USA) 69-73, Matt Jones (AUS) 72-70, Corey Conners (CAN) 70-72, Rafael Cabrera (ESP) 68-74, Park Sang-hyun (KOR) 74-68, Lee Hyung-joon (KOR) 71-71, Branden Grace (RSA) 71-71
143 - K.J. Choi (KOR) 69-74, Sergio Garcia (ESP) 73-70, Jhonattan Vegas (VEN) 73-70, Lucas Glover (USA) 74-69, C.T. Pan (TPE) 74-69
144 - Scott Piercy (USA) 72-72, Brooks Koepka (USA) 69-75, Kang Sung-hoon (KOR) 72-72
145 - Chez Reavie (USA) 68-77, Adam Schenk (USA) 71-74, Adam Long (USA) 73-72, Luke List (USA) 70-75, Joel Dahmen (USA) 74-71, Vaughn Taylor (USA) 74-71, Billy Horschel (USA) 71-74
146 - Keith Mitchell (USA) 74-72, Troy Merritt (USA) 74-72, Ham Jung-woo (KOR) 74-72, Chang Yi-keun (KOR) 73-73
147 - Abraham Ancer (MEX) 73-74, Won-joon Lee (AUS) 74-73, J.B. Holmes (USA) 70-77, Danny Willett (ENG) 76-71
148 - Brian Stuard (USA) 72-76, Michael Kim (USA) 76-72, Jazz Janewattananond (THA) 74-74, Moon Kyong-jun (KOR) 76-72
150 - Bae Yong-jun (KOR) 78-72
151 - Lee Tae-hee (KOR) 74-77, Whee Kim (KOR) 76-75, Max Homa (USA) 69-82, Chesson Hadley (USA) 75-76, Matthew Wolff (USA) 73-78