Thomas Bjorn posted a 67 to join Johan Edfors, Ivo Giner, Terry Pilkadaris, Wen Teh Lu and Keng-Chi Lin in a tie for fifth place at 11-under-par 133.
Quiroz stumbled out of the gate at Yalong Bay Golf Club with a bogey at the 10th, his first. He recovered well, however, with back-to-back birdies from the par-5 13th and an eagle at the par-5 16th.
He added a birdie at the par-4 17th and another at the par-4 18th to make the turn at 12 under. Quiroz cooled off on the front nine with six consecutive pars before he picked up a birdie at the par-4 seventh.
Quiroz then closed with a birdie at the par-5 ninth to hold the outright lead heading into the weekend.
'I have to take it as just one more day on the golf course and will just try and play well and enjoy myself,' said Quiroz.
Montgomerie needs a strong finish this week to move back into the top 50 in the world in his bid for a spot in the Masters, and the Scot did well early on with a birdie at the par-5 first.
The 41-year-old then birdied the par-4 seventh and moved to 8 under with a birdie at the par-4 10th. Montgomerie eagled the 13th and kept on rolling with a birdie at the 16th. He capped off a round of 65 with a birdie at the last.
Casey was part of a four-way tie for the lead after the opening round and the Englishman sought to regain that position in round two. He struggled early with a bogey at the second but got that shot back with a birdie at the very next hole.
The 27-year-old birdied the seventh and got within three of the lead with an eagle at the par-5 ninth. Casey was cruising on the back nine with a birdie at the par-4 10th, but a bogey at the 13th dropped him back to 11 under.
Casey responded with a birdie at the par-4 17th en route to a round of 68.
Choi played the back side first and birdied three in a row starting at the 12th to get to 10 under. He dropped a shot at the sixth but came back with three consecutive birdies from the seventh for a round of 67.
Paul McGinley, Michael Campbell, Emanuele Canonica, Chawalit Plaphol, Wen-Tang Lin, Edward Loar, Joong Kyung Mo, Steven Jeppesen and Ross Bain are tied at 10-under-par 134.
The 36-hole cut fell at 6-under-par 138 with 72 players qualifying for the weekend. Ian Woosnam missed the number at 2-under-par 142.