The little-known Thai qualifier, who only has one top 10 Asian Tour finish in his career, also had two bogeys at the Hong Kong Golf Club.
"I played according to my game plan and was really pleased that everything went the way I hoped it would," said the 38-year-old Duangdecha, who nearly equaled the course record of 61 by Simon Yates in 2005 but bogeyed his final hole amid fading light.
Eyeing the European Tour money title, Lee Westwood and Rory McIlroy both shot 66. The duo are the top two money winners in the Race to Dubai, and the Hong Kong Open is the penultimate tournament before the season-ending event in the United Arab Emirates next week, which has a $7.5 million bonus prize pool.
Westwood, who had six birdies and two bogeys, said he was happy with his performance considering it was his first competitive round in Hong Kong.
"It's all right learning a course tee to green -- it's pretty obvious how you play tee to green -- but on the greens, it's such an advantage having played a few tournaments round here," the fourth-ranked Englishman said. "So to start with 4-under 66, I'm delighted."
McIlroy, who had five birdies and a bogey, said he needed to step up his game a notch to have a shot at the title.
"I'm not sure if three more 66s will be good enough so I might need to go a little better over the next three days," the Northern Irishman said.
Also, Oliver Wilson (70) had three birdies but double-bogeyed the 18th. Ian Poulter (68) was tied for 40th, and eight-time European Tour Order of Merit winner Colin Montgomerie (69) was tied for 65th.