The popular duo were placed tied 12th, five shots off the pace set by Robert Allenby of Australia at The Mines Resort and Golf Club.
Indian stalwart Jeev Milkha Singh, a two-time Order of Merit winner, was a further shot back in tied 19th place in the event sanctioned by the Asian Tour and PGA Tour.
Siddikur was four-under going into the last hole before an errant approach shot landed in the greenside rough which forced him to settle for a bogey.
"I should have made the up-and-down there but my chip fell short off the green. I'm still happy with my score because there's still a long way to go," said Siddikur, who finished tied 11th in last year's CIMB Asia Pacific Classic, Malaysia.
Siddikur added he was brimming with confidence after playing with four-time PGA Tour winner Mark Wilson of the United States.
"I'm feeling even more confident now because my driving distance compared to Mark was about 10 yards different. The distance of our iron shots were almost the same. It goes to show that the standard of our game is almost the same and if they can play on the PGA Tour so can I," said Siddikur, the first Bangladeshi to win on the Asian Tour.
Thongchai, playing in his fifth consecutive week, struggled with fatigue but managed to return with five birdies against two bogeys at The Mines Resort.
"My body is a bit tired. I had a good sleep in the last two days. I'll try to have a good rest today and get myself ready," said the three-time Asian Tour Order of Merit winner.
Singh, who is recuperating from a back injury, turned in 34 before adding two straight birdies from the 16th hole against one bogey on 14.
"I'll take a two under especially after a two-week break. My back is still not 100 per cent. I need to release the club more as I'm still holding back with the club. I feel as the weeks go by, it'll get better," said Singh.