SEOUL, 23 May 2015 (AFP) - Choi, the overnight leader, shot a two-under-par 70 to maintain his top spot on the leaderboard -- but only just.
One shot behind was 19-year-old compatriot Wang Jeung-Hun who fired a course record 63 to surge into joint-second place with another Korean Kang Sung-Hoon, who carded a 69.
Just one more stroke back was 21-year-old Lee Soo-Min who also charged through the course with a 63.
"I didn't watch the leaderboard and it was probably just as well looking at those scores," said Choi, who turns 31 next week.
Wang and Lee's rounds beat the previous course record of 64.
It was a consistent performance by Choi, who is enjoying only his third tournament back on tour since finishing a 20-month stretch of military service, which is mandatory for every able-bodied South Korean male between the ages of 18 and 35.
Wang's record round included an eagle, eight birdies and one bogey and, despite his youth, he has already shown he is capable of competing for top place finishes.
Wang triumphed in the Mission Hills Haikou Open last year by cruising to a 10-shot victory on the PGA Tour China.
Defending champion Kim Seung-Hyuk from Korea carded a 67 on Saturday to leave him three off the lead.
Leading second-round scores (South Korean unless stated):
206 - Choi Jin-Ho 68-68-70.
207 - Wang Jeung-Hun 70-74-63, Kang Sung-Hoon 69-69-69.
208 - Lee Soo-Min 73-72-63, Cho Ming-Yu 70-71-67.
209 - Kim Seung-Hyuk 74-68-67.
210 - Kim Kyung-Tae 73-69-68, David McKenzie (AUS) 69-72-69, Park Jae-Bum 72-69-69, Kim Seng-Yong (KOR) 69-72-69.