JEJU • Justin Thomas claimed he had an "easy 63" as he made the most of benign conditions to shoot a nine-under 63 yesterday for a two-stroke lead at the halfway stage of the CJ Cup on the South Korean resort island of Jeju.

The American's blemish-free second round also put him in contention to win the PGA Tour event for the second time in three years, after also winning the inaugural edition in 2017.

He birdied his first four holes, picked up another shot on the eighth before sinking four more birdies on the back nine at Nine Bridges Golf Club to reach a 13-under 131 total.

Overnight leader and local favourite An Byeong-hun shot 69 to tie for second spot with South Korean-born New Zealander Danny Lee (66) on 11 under.

"That was, I would say, overall one of the better rounds I've played in a while," Thomas, 26, said after equalling his own best round at the US$9.75 million (S$13.3 million) event.

"I didn't do anything unbelievably, it was a very stress-free day. I think I hit every green and made a couple of putts early...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, but hopefully we'll be able to keep some of that going for the weekend."

He rated the 63 he registered two years ago at the tournament as better than yesterday's round.

"I'll take 63 on any conditions on any course any day, but it was very, very benign out there," he said.

"There just wasn't very much wind, which made it a lot easier to score. I felt like when I had control of the ball like I did today, it made it quite a bit easier."

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 the South Korean.

American Jordan Spieth (65) and Argentina's Emiliano Grillo (66) were tied for fourth, four strokes behind Thomas in the first tournament of the PGA Tour's three-event Asian swing.

The Tour next heads to Japan for the inaugural Zozo Championship before moving to China for the WGC-HSBC Champions.

Brooks Koepka's title defence appeared to be wilting, with the American world No. 1 recording a three-over 75 in the second round and falling 13 strokes behind the leader.

The highlight of the day was when Phil Mickelson came within a whisker of an amazing new chapter in his storied career when he was a fraction from a hole-in-one - on a par four.

The five-time Major winner hit the pin at the 353-yard 14th hole, with the ball finishing just inches from the cup for a brush-in eagle.

Had it dropped, it would have been only the second ace at a par four in PGA Tour history.

A hole-in-one is extremely rare and an albatross - three under par on one hole - is even rarer. But a combination of both on a par four is virtually unheard of.

The only one in the history of the Tour was made by Andrew Magee on the 17th hole at Scottsdale in Phoenix, Arizona, in January 2001.

Mickelson, who carded a second-round 72 to add to his opening 70, lies 11 strokes off the lead.

REUTERS, AGENCE FRANCE-PRESSE