Kevin Stadler, who shared the first-round lead with Adam Scott, posted a 3-under 69 to take second place at 11-under-par 133.
Scott, the defending champion, is two strokes further back at minus-nine after a 71. He was joined in a tie for third place by fellow Australians, Tony Carolan and Richard Green, who each shot 69 on Friday.
Choi started his second round on the back nine at The Vines Resort & Country Club. He posted his first birdie on the par-4 12th to move to 8-under. He then birdied the 14th for the second straight round.
The Korean climbed to 10-under as he birdied the par-3 16th. Choi posted the fourth birdie of his opening nine holes at the 18th, to make the turn at minus-11.
Choi dropped in a birdie effort on the second. He came right back with a birdie on the par-5 third. The 35-year-old parred his final six holes to close at 13-under.
'My focus today was my iron shots and my putting,' Choi said. 'The greens are fast here, which is good because I usually like fast greens. I've been feeling good with my putter and I haven't missed many shots. I'm just playing good and making birdies.'
Stadler, who is playing this week on a sponsor's exemption, got his round going with a birdie on No. 10. He cruised to seven consecutive pars from there.
The 26-year-old eagled the par-5 18th to make the turn at 11-under. Stadler stumbled to just his second bogey of the event at the par-4 fifth. He got that stroke back when he birdied the ninth, his last.
'It was tough out there in the heat, but I hit it pretty well without making as many putts as yesterday,' said Stadler, who with the field battled temperatures that soared into the 100s. 'I still feel positive about things.'
Scott, who won this event last year when it was played at Pine Valley Golf Resort and Country Club in Beijing, China, stumbled to back-to-back bogeys from the 11th. He got those strokes back at consecutive birdies from the 17th. Scott birdied the par-4 second to go with eight pars on his second nine.
'I had heaps of chances -- probably more than Thursday -- but hardly made any putts,' Scott said. 'All things considered, I am still in a pretty good position. I just need to hole a few putts Saturday to get myself right into contention.'
Australian Gavin Flint fired an 8-under 64 to jump into a tie for sixth place at 8-under-par 136. He was joined there by Francesco Molinari (65), Stephen Leaney (66), Ian Poulter (66), Peter Senior (68), Ross Fisher (67) and Scott Gardiner (66). Robert Allenby, Paul Casey and Stephen Gallacher are one stroke further back at minus-seven.
The round of the day was posted by Australian Gary Simpson. He carded a course-record tying 9-under 63. He notched eight birdies, a bogey and an eagle in his round. That 63 helped him jump from a tie for 130th place into a share of 16th.
Fred Couples, the 1995 champion, posted a 5-under 67. He is one of seven players that stand alongside Simpson at minus-six.
The cut line fell at two-under-par 142 with 65 players advancing to the final two rounds. Several big-name players will not be around for the weekend.
Among those missing the cut were - two-time U.S. Open champion and 2002 Johnnie Walker winner Retief Goosen (even-par 144), 1993 runner-up Colin Montgomerie (144), European Ryder Cup captain and 1996 winner of this event Ian Woosnam (145) and reigning U.S. Open champion Michael Campbell (146).