Wi-joong totaled 8-under 134, a shot ahead of Poulter (66) and Kim Dae-sub (68).
Opening-round leader Anthony Kim, a star of the U.S. Ryder Cup, followed his first-round 64 with a 73 to trail the leader by three shots.
Wi-joong had four birdies and an eagle despite early fog, which delayed his start at the Woo Jeong Hills Country Club. Forty-five players will complete their second round on Saturday morning.
Wi-joong used the delay to stay loose.
It gave me an opportunity to warm up, Kim Wi-joong said. It felt much better than actually having to tee off in the early morning.
Poulter, runner-up in the British Open and Europes best player at the recent Ryder Cup, had the best round of the day.
To shoot a five under today is good, Poulter said. I am a little disappointed with the two bogeys. I three putted both the holes. I could have been a couple of shots better.
American Kim struggled with a double-bogey and two bogeys.
I had no control over my distance, he said. I have to tighten up my driver now. It was not a good round, but I still have a chance heading into the weekend rounds.