An, who didn't have any bogey through his first 54 holes, held steady with three birdies on the back nine and made the turn at 20-under. And playing in a group ahead, Poston made a run with three birdies and an eagle over his first nine holes to get to 19-under.

An had a birdie at the 10th, but Poston caught him with birdies of his own at the 10th and the 14th.

The South Korean blinked first at the 15th, where he found trouble off the tee. His drive landed in fescue to the left of the fairway, and An made a bogey on the par-5 hole. It was his first bogey of the tournament.

An bounced back with a birdie at the next hole, but a putt for another birdie at the 17th stopped about a foot short of the cup.

Poston picked up another birdie at the 15th and took the clubhouse lead at 22-under after four rounds of bogey-free golf. An still had a chance to force a playoff with a birdie at the 18th but pulled his tee shot way left en route to his second bogey of the day.

An put his second shot short on the front portion of the green and ran his long birdie attempt well past the cup. He missed the comebacker for par, which dropped him into third place behind Poston and Webb Simpson, who closed with a birdie at the final hole.

An, the 2015 European Tour Rookie of the Year, has three runner-up finishes on the US circuit. This is his fourth year on the PGA Tour. (Yonhap)