Former world No 1 Jason Day is five shots off the lead after an opening 68 which included eight birdies and a double-bogey seven on the 18th, with Tour Championship victor Xander Schauffele a shot further back.
2009 US Open champion Lucas Glover is in a tie for second place alongside Scott Brown of the United States of America on eight under par, whilst Patrick Reed and Marc Leishman are both in contention on six under par.
Jhonattan Vegas and Charles Howell were among five players at 5 under.
Thomas was surprised with his round because his day didn't begin well.
"I start my military service in a month, so for a long time I hadn't actually played any golf".
It was a disappointing day for FedEx Cup champion Justin Thomas as he slipped from top of the leaderboard to joint-fourth alongside Cameron Smith as he followed up his impressive first-round 63 with a two-over 74 on Friday to sit two shots off the pace.
"And that was pretty much most of my round".
He was at 29 through his first nine holes, which led Thomas to feel a little disappointed that he didn't post a lower round than 63.
The 24-year-old American's second nine holes were sedate by comparison, with just three birdies and a bogey, though he finished on a high by picking up a stroke on the ninth.
Thomas won the CIMB Classic in Malaysia in 2015 and 2016 and said he is comfortable playing overseas.
Elsewhere, Korea's Eun-Hee Ji opened with a six-under-par 66 to take a three-shot lead after the opening round of the LPGA Tour's Swinging Skirts LPGA Taiwan Championship.
"If I would have parred my last hole, I would have had a pretty sour taste to my mouth walking off the golf course".
Former world No. 1 Jason Day of Australia is five strokes adrift after carding a four-under 68 at the US$9.25 million (S$12.5 million), 78-player event.
Seventeen Korean golfers are in the field with Whee Kim (68) and K.J.