INCHEON, May 15, 2014 (AFP) - The 28-year-old was eight-under on the par-four sixth hole after starting on the back nine when an overconfident swing saw him slice out of bounds.
He compounded the error with a three-stroke putt.
The leader closed with an improbable birdie, a 50-footer from the fringe -- the last of many during the round.
Lee Tae-Hee (68) was second, while seven others -- including OneAsia winner Kim Kyung-Tae -- were at two under.
Kim, a OneAsia and Japan tour regular without a win since turning professional in 2005, birdied his second and the last three holes of the first nine.
"I was playing very well off the tee and with my approach shots," he said. "It was one of those days where everything was going well."
Four birdies in the first five holes of his back nine put him in sight of the course record of 63, until the disastrous sixth.
"It was overconfidence," he said.
Defending champion Matthew Griffin and South Korean PGA star K.J. Choi -- a three-time winner of the $1 million event -- settled for matching 73s under windy conditions in the afternoon.
"I will need to judge the greens a bit better tomorrow," Choi said.
Leading scores (South Korea unless stated):
66 - Kim Seung-Hyuk
68 - Lee Tae-Hee
70 - Kim Kyung-Tae, Lee Kyoung-Hoon, Lee Dong-Min, Hur In-Hoi, Kim Do-Hoon, Lucas Lee (BRA), Baek Seuk-Hyun