The 34-year-old Thai, who lost his Asian Tour card last season and only made the field because of his career money earnings, started the day with four straight birdies and picked up three more shots to be at seven-under 135.
Spaniard Garcia was a shot behind and one-under after completing only eight holes in his second round.
Chapchai's compatriot Tirawat Kaewsiribandit (70), South Africa's Shaun Norris (67) and American Kurt Kitayama were also alongside Garcia on six-under when play was suspended and later abandoned for the day.
"To finish the round seven-under-par was beyond my expectation. Before coming to this tournament, I tried to adjust and fix my swing flaws with my coach," Chapchai told reporters.
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"Today, I was attempting to play the same way as I did during practice and follow my coach's advice.
"For this tournament, I didn't think too much and I didn't set any target. I'm just enjoying the experience and trying to get used to my new golf swing as well as getting a better swing rhythm."
"For the next two days, I will do the same as I did today and don't think much about the result."
India's Shiv Kapur shot a 67 to lie just two shots off the lead alongside Americans Jarin Todd (66) and Casey O'Toole, who could complete only six holes in his second round.
(Reporting by Sudipto Ganguly in Mumbai; Editing by Toby Chopra)
