YANGON, Feb 21, 2013 (AFP) - The pair, who finished on eight-under-par 64, are one shot ahead of fellow countryman Prom Meesawat, who is seeking a long-awaited second Asian Tour victory, and Joonas Granberg of Finland.
Another Thai, Thanyakon Khrongpha, shared fifth place with Australia's Darren Beck and South Koreans Hwang In-choon and Young Nam at the $300,000 event, the curtain-raiser for the 10th Asian Tour season.
Reigning Asian Tour number one Thaworn Wiratchant of Thailand, who hit the first drive to mark the occasion, was on 71 along with title holder Kieran Pratt of Australia.
Kiradech was six-under through eight holes but his confidence took a jolt when he missed a 10-foot birdie putt on the 10th hole. He gradually recovered and took advantage of the par fives, where he birdied 14 and 18.
"I missed a big putt on the 10th hole," said Kiradech, in comments supplied by the organisers.
"I had a 10 feet uphill straight putt but I pulled it. After that I lost some confidence with the putter. I was lucky to get two birdies in the next par fives."
Chapchai was even-par after five holes but recovered with eight birdies in his last 13 holes to share the lead.
Leading scores after round one at the par 72 course:
64 - Chapchai Nirat (THA), Kiradech Aphibarnrat (THA)
65 - Joonas Granberg (FIN), Prom Meesawat (THA)
66 - Thanyakon Khrongpha (THA), Hwang In-Choon (KOR), Young Nam (KOR), Darren Beck (AUS)
67 - Mardan Mamat (SIN), Choo Tze-huang (SIN), Chawalit Plaphol (THA), Antonio Lascuna (PHI), Hung Chien-yao (TPE), Chan Shih-chang (TPE), Sam Brazel (AUS), Carlos Pigem (ESP), Hu Mu (CHN), Rikard Karlberg (SWE), Baek Seuk-Hyun (KOR), Gunn Charoenkul (THA), Lionel Weber (FRA)