Justin Thomas earned a two-stroke lead with a sizzling performance at the halfway stage of the CJ Cup in South Korea on Friday.
American star Thomas used a bogey-free nine-under-par 63 to surge to the top of the leaderboard following the second round.
Thomas started his day with four successive birdies and finished with nine in total, including one at the last as he catapulted himself eight positions into the lead at 13 under through 36 holes.
Winner of the PGA Tour tournament – which is being played outside of the United States this year – in 2017, Thomas leads An Byeong-hun and Danny Lee heading into the third round.
Local hope An (69) and New Zealand's Lee (66) are tied for second at the Nine Bridges Golf Club on Jeju Island.
An carried a one-shot lead into the second round but the South Korean lost his spot at the front of the queue following two bogeys and five birdies, while Lee improved to 11 under but he could have ended the day a stroke better off if not for a bogey at the 18th.
Emiliano Grillo (66) and former world number one Jordan Spieth (65) are four shots off the pace ahead of Saturday's penultimate round.
Spieth was a big mover at the CJ Cup, with the three-time major champion climbing 25 positions into a tie for fourth, thanks in part to back-to-back birdies to end the day.
The American almost aced the 17th after his tee shot fell just short of the hole, highlighting a fine display from Spieth.
Jason Day struggled as the Australian's second-round 73 saw him drop down to five under, while defending champion Brooks Koepka also had a round to forget.
Looking to defend his title, Koepka was unable to back up his opening-round 69 after shooting a three-over-par 75 – a round that included five bogeys and left him even par.