Scott Piercy holds a one-shot lead after the second round of the CJ Cup in Korea, with Brooks Koepka in second place as he chases the world number one ranking.
Three-time major winner Koepka can reach top spot in the rankings for the first time with victory. A solo second-place finish would also be enough assuming current world number four Justin Thomas doesn't win the event.
American Piercy picked up four birdies on the front nine as he shot a 65 to sit nine under, one stroke ahead of compatriot Koepka in the £7.2million event.
An eagle three at the 18th helped Koepka also sign for a seven-under round - which could have been better but for a bogey at the par-five ninth.
Defending champion Thomas, knowing a win would lift him back to world number one, improved slightly after his opening 73 to move to one under after a second round of 70 - but sits eight shots off the lead.
Ireland's Graeme McDowell had a mixed round, starting with a bogey before going on to hit two more and three birdie for a 72, leaving him on -1 for the tournament.
Ian Poulter, who helped drive Europe to Ryder Cup success, is in fourth place after his 69.
The Englishman had struggled early on after dropping two shots in the opening three holes before recovering them to head into the turn at even par for the day.
Further birdies on the 12th, 14th and at the par-five last helped keep Poulter right in contention heading into the weekend.
Sweden's Alex Noren raced up the leaderboard with another blemish-free seven-under round to also sit at five under.
Overnight leader Chez Reavie dropped back to third after his 70, with successive bogeys over the closing holes proving costly.
Friday's best round came from Brian Harman, the American producing a strong finish with four straight birdies to sign for a 64 and climb up to a tie for sixth place, some five shots off the lead.
England's Paul Casey hit a hole-in-one at the par three seventh, but then sank four bogeys in his 73 to drop down to tied for 30th.
Former Masters champion Danny Willett had been just a shot off the lead at the start of the day. However, a five-over 77 - which included a double-bogey at the fifth - saw him fall out of contention.