Still, his 66 left him at 15-under 201, one clear of Bjerregaard (70), South Korea's An Byeong-hun (66) and Sergio Garcia of Spain (67).
Thongchai has won seven times on the European Tour but victory on Sunday at the $7 million event, the penultimate in the Tour's Final Series, would easily be his biggest.
"Tomorrow is a very big day, if I play well like this and keep performing like this, I have a good chance," the former paratrooper said. "But golf, you never know.
"I think my game plan this week is good. Hit irons well, working on putting, and that's why what gives me very good confidence."
The three-time Asian Tour Order of Merit winner, who won the European Open in September, picked up five birdies in his first eight holes to move ahead of Denmark's Bjerregaard, who had started the day with a three-shot lead.
The two dovetailed in and out of the lead for most of the day, with Bjerregaard, chasing his first win on Tour, dropping a costly shot on 16.
Garcia had led the field after the first day but fell off the pace in Saturday's third round before a rousing four birdies in five holes from the 13th put him in position to land a first title of the year.
"It was nice to be able to finish like that and give myself a shot tomorrow because I was getting a little bit far behind," the world number 11 said.
An, chasing rookie of the year honours following his win at the flagship PGA Championship at Wentworth in May, carded a bogey-free round to grab a spot in the final group on Sunday with Thongchai and Garcia.
Sweden's Kristoffer Broberg shot a 64 to move up to 13-under and fifth place, one ahead of a high calibre group that included Henrik Stenson (65), American Patrick Reed (64) and English trio Ross Fisher (67), Paul Casey (68) and Ian Poulter (68).
Australian Marcus Fraser produced a grandstand finish to his round by holing a five wood from 231 yards at the last for an eagle two, completing a back nine of 29 and round of 65 to move into a share of 11th on 11-under.
(Writing by Patrick Johnston in Singapore; editing by Amlan Chakraborty)
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