Open champion Els was tied fourth, four strokes behind Thongchai, after carding five birdies against three bogeys for a two-under-par 70 at the Royale Jakarta Golf Club.
"Today was a bit of a jet-lag day for me. I was trying to get some energy going. Hopefully, I will have more energy tomorrow. I think my game is all there. I just need to get a bit sharper mentally," the big South African said.
"I was hoping for a better score but I didn't quite get it. My game is not too bad. I just need to eliminate some really small mistakes.
"This is the type of course where you can get a little bit aggressive. I'm hitting the ball quite nicely so I like to stay aggressive. If I get myself in position, I will go for flags and get my putts in," the four-times Major winner said.
A confident Thongchai, three-times Asian Tour Order of Merit winner, hit a blemish-free seven-under-par 65 to move two shots ahead of Australian Scott Barr.
"I hope I can keep my confidence like this in the next few days. If I drive, hit and putt like today then I think I have a good chance to win the tournament," the Thai, who has been joined by his wife and two sons in Jakarta, said.
"My putting wasn't that good in the last few months but it is coming around now.
"I played very solid and I have no complains at all. I think I have a chance to go even lower in the next few days. I'm only halfway there. There are many strong players here but getting off to 10-under after two rounds gives me a good chance."
Wade Ormsby of Australia was three shots behind the leader and Hung Chien-yao of Chinese Taipei and three more Australians, Kieran Pratt, Jake Higginbottom and Scott Hend, shared fourth place with Els.
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