Piercy leads, Koepka lurks at PGA tour event in S. Korea

The 39-year-old Piercy defied his world ranking of 252 to fire a bogey-free seven-under-par round of 65, moving to nine under for the $9.5 million US PGA tournament.

But Koepka, US PGA Tour Player of the Year, sits just one back after his own 65. He finished with a flourish, picking up an eagle at the par-5 18th following a booming tee shot.

"I feel like I struck it really well," Koepka said. "I putted really well. My iron play was very consistent and I gave myself good looks. 

"If you can put it on the fairway and give yourself a good look at birdies, these greens are so good you're going to make a few putts."

The conditions at the Club at Nine Bridges on the resort island of Jeju were far gentler than Thursday's blustery first round, when just 18 players broke par. A total of 44 did so on Friday, led by Brian Harman's 64.

Koepka -- who averaged 313 yards off the tee last season, ranking eighth on the tour -- said he capitalised on the benign conditions with length.

"If there's no wind, you can definitely take advantage of it," he said. "It brings the driver into play for me a bit more and I can fire away."

Piercy, who tied for 27th at the CIMB Classic in Malaysia last week, said his putter "showed up".

"I made some putts of pretty good lengths," he added. "Hopefully, the putter will stay there."

Overnight leader Chez Reavie carded 70 to move to six-under and slipped to third place, while Ryder Cup teammates Ian Poulter and Alex Noren were both at five-under through 36 holes, the Swede matching Piercy with a bogey-less 65.

But England's former Masters winner Danny Willett -- who was joint second after the first round, still seeking a first tournament win since taking the Green Jacket at Augusta National in 2016 -- had a nightmare day.

He dropped five shots in six holes from the second to the seventh -- including a double bogey on the par-4 fifth -- to card a five-over 77 and slump 54 places on the leaderboard.