Liang Wenchong of China, Spain's Pablo Larrazabal and Jamie Donaldson of Wales shared third after carding six-under-par 66 over the 7,326-yard Gary Player design at the Suzhou Jinji Lake International Golf Club.
Hong Soon-sang of Korea shot a 67 to tie for sixth with 2008 champion Damien McGrane, fellow Irishmen Peter Lawrie and American Corey Pavin, who played with fellow Ryder Cup captain Colin Montgomerie (72) and Thongchai.
Yang Yong-eun, Asia's first Major winner, and Sweden's Henrik Stenson each shot 68 to lie in a share of 10th.
Thongchai, 40, shot his remarkable bogey-free round despite terrible preparation, including a left-arm injury that caused the former army paratrooper to withdraw in the second round of the Masters last Friday.
"I haven't hit eight-under for a long time. It was perfect, especially as I only practised for two holes on Wednesday and quit because of the rain. My caddie had to walk the rest of the course and gave me some great advice today," said Thongchai, a three-time winner of the Asian Tour Order of Merit.
"My arm feels better. I had an injection on Saturday, three days' rest and was even considering withdrawing this week, but it felt better and it's now about 70 per cent. Everything was perfect today - my driving, my irons, my putting."
Thongchai picked up his first shot on the fifth, bagged a hat-trick of birdies from seven to nine, then chipped in for eagle on the par-five 14th. Birdies on 16 and 17 put him two ahead of clubhouse leaders Liang and Larrazabal.
Kim, 21, also eagled the 14th as he carded a 64 for the third straight week. The Busan resident closed with an eight-under round to finish joint-third at OneAsia's season-opening Luxehills Chengdu Open before firing another 64 in his first Korean Tour victory at last week's Tomato Savings Bank Open.
"I played really well and am in good form. I eagled the 14th with a 240-yard utility club approach, then holed a long putt from 15 yards (45 feet)," said Kim, who only turned 21 last Sunday. "I'm not expecting to shoot a 64 again, but we'll see."
Liang teed off on the 10th and fired off three straight birdies, picking up further shots at 14 and 16 before recording his sole bogey on 18 after lipping out with his second putt. He also birdied three and five.
The Chinese number one is aiming to secure a third successive OneAsia title in China after winning Luxehills earlier this month and the Midea China Classic last October.
"I had a great start as I started with three straight birdies, so that put me in a confident mood. It was still cold today, so I was pleased to score as well as I did," said the 31-year-old, who has long held the dream of winning his national Open.
"I've prepared well for this tournament, so I knew what to expect in terms of the weather."
Larrazabal was five-under after playing the front nine. The 26-year-old, who won the Open de France Alstom in 2008 when he was also named Sir Henry Cotton Rookie of the Year, picked up his solitary bogey on 13 before responding with birdies on 14 and 17.
"I was wide awake for my tee-time at 6.40 as I only had three hours' sleep because I still have jet-lag," said the livewire Spaniard, whose elder brother Alejandro is carrying his bag this month.
"I played the front nine perfectly, but the back nine has lots of high, links-style bunkers. I'm hitting the ball great, as well as I've ever hit it, and I'm putting great."
Chinese amateur Hu Mu, 20, and 18-year-old Korean Noh Seung-yul, winner of last month's Malaysian Open, were among a group of players to shoot 69.
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