Jeju - World No 4 Justin Thomas saw his three-shot overnight lead quickly blown away in gusting winds in Friday's second round of the CJ Cup on Jeju Island.
American Luke List leads the elite 78-man field at the US PGA Tour's first-ever event in South Korea after he compiled one of the few bogey-free cards, adding a 67 to his opening round 68 to reach nine-under par.
"Any time you go bogey-free you're really happy about it," said List, who leads the $9.25 million tournament by a shot. "The wind was tricky today but I was able to save par a lot and then played solid on the back nine."
Nine-under was the same mark on which FedEx Cup champion Thomas had started the day after a sizzling 63 in benign conditions at the Nine Bridges layout on Thursday. He was 11 shots worse as he slumped to a 74 Friday.
The 23-year-old PGA Tour Player of the Year dropped three shots on the front nine which took him 39 strokes, having made the turn in 29 on Thursday.
He did manage a first birdie at the 10th but took six at the 18th to come home in 35 and slide down to a share of fourth place on seven-under par with Australia's Cameron Smith, who had a round of 68.
"It was really difficult out there," said Thomas. "I really fought back hard to get it to one-over going into 18, which was kind of our goal after 11 or 12 holes.
"When we got there, it switched into the wind. It has nothing to do with the six shots it took me to play the hole and that sums up the day. It wasn't the best."
Ahead of that pair on eight-under are the 2009 US Open champion Lucas Glover, who also shot 67, and fellow American Scott Brown who added a two-under 70 to his opening 66.
"It was whipping around pretty good the first four or five holes on the back," Glover said of the varying wind whistling around the nearby Halla Mountain.
"But last three or four holes, it was out of the other direction and then swung back so it was difficult to pick."
England's in-form Paul Casey started on the 10th and got into contention as the only player to eagle the 18th on both Thursday and Friday - but in very different conditions.
"Yesterday it was a driver over the corner and a soft eight-iron," Casey, who finished seventh in Malaysia last week, told AFP. "Today was a fully ripped three-wood. Then another ripped three-wood from 275 yards to about five and half feet."
But the world number 14 then blew all his good work at the eighth, his 17th hole. "I 'ballsed' it up," Casey said frankly. "Hit a sand wedge in the bunker and made double bogey."
His 143 total means a share of 30th place, but the event remains wide open with two rounds to play and 20 players within five of the lead - especially if the wind gets up at the weekend.
Leading scores after the second round of the US PGA Tour's CJ Cup at Nine Bridges, Jeju Island, South Korea (par 72, USA unless stated):
135 Luke List 68-67
136 Lucas Glover 69-67, Scott Brown 66-70
137 Cameron Smith (AUS) 69-68, Justin Thomas 63-74
138 Marc Leishman (AUS) 66-72, Chez Reavie 66-72, Patrick Reed 66-72, Whee Kim (KOR) 68-70
139 Branden Grace (RSA) 72-67, Charles Howell III 67-72, Thomas Pieters (BEL) 71-68, Ollie Schniederjans 67-72, Kyle Stanley 68-71
140 Brian Harman 68-72, Anirban Lahiri (IND) 69-71, Pat Perez 69-71, Robert Streb 69-71, Nick Taylor (CAN) 68-72
141 Rafa Cabrera Bello (ESP) 71-70, Stewart Cink 71-70, Hudson Swafford 68-73, Harold Varner III 69-72