Thomas Bjorn shot a 68 to join Terry Pilkadaris at 15-under-par 201. Ivo Giner was one shot further back at 14-under-par 202 along with Wook-Soon Kang, Keith Horne and 36-hole leader Alex Quiroz.
Montgomerie picked up a pair of birdies over his first three holes but trouble loomed on the front nine at Yalong Bay Golf Club. The Scot bogeyed three in a row starting at the par-4 fourth to tumble down the leaderboard, but he was able to counter with a birdie at the par-4 seventh.
The 41-year-old added a birdie at the par-5 ninth and moved to 14 under with a birdie at the par-5 13th. Montgomerie, who can move back into the top 50 in the world with a win on Sunday, closed with birdies on each of his last two holes to grab a piece of first with one round to play.
Casey, who was part of a logjam atop the leaderboard after the opening round, collected two birdies over his first nine holes. He then birdied the 13th to get to 15 under but gave that shot back with a bogey at the 15th.
The Englishman responded with a birdie at the 16th and added a birdie at the last to complete a round of 68.
Plaphol was one of four co-leaders after round one and got back to form on Saturday with a birdie at the sixth and an eagle at the short par-4 seventh.
The 30-year-old from Thailand picked up another birdie at the ninth. Plaphol struggled to a bogey at the 11th but recovered with three birdies over his last seven holes for a round of 66.
Bjorn bogeyed the fifth but tallied an eagle at the ninth to make the turn at 12 under. The Dane then birdied the 14th and added back-to-back birdies starting at the par-5 16th to finish one shot back.
Paul McGinley, Johan Edfors, Amandeep Johl, Wen-Tang Lin, Edward Loar and David Bransdon share 10th place at 13-under-par 203. Michael Campbell, Thaworn Wiratchant, Joong Kyung Mo, Steven Jeppesen and Hendrik Buhrmann followed at 12-under-par 204.