His 36-hole total, 6-under 138, leads both Austria's Markus Brier (73), the front-runner after Thursday's first round, and Thailand's Chapchai Nirat (71).
A birdie at No. 14 gave Choi a brief three-stroke advantage, but he closed with a bogey and three straight pars.
"I three-putted the first hole and then had to work hard for the rest of my round," he said. "I'm going to take that (memory) through the rest of the tournament."
Brier bogeyed three of the first six holes and his round included only two birdies.
Spain's Gonzalo Fernandez-Castano (70) and Ireland's Paul McGinley (67) share fourth place at 3-under 141.
McGinley's 5-under 67 was the day's low round.