Iwai set the Siam Country Club in Pattaya alight with seven birdies in her front nine and three more on the way home to claim the lead from Sweden's Maja Stark.
Thai veteran Moriya Jutanugarn sits in third on six-under. Iwai's twin Chisato had an opening round to forget, carding four bogeys on her way to even par and a share of 40th place.
Akie Iwai, in her debut season on the top women's tour after a successful spell on the Japanese circuit, said she was pleased with a good all-round performance.
"Today I did shoot well and putted well, both," she said.
Two-time Solheim Cup star Stark said she was happy to be driving better than at the Founders Cup at Florida's Bradenton Country Club this month, where she missed the cut.
"I felt like in Bradenton I had a couple of big misses off the tees that really ruined my scores," the 25-year-old said.
"Finally got more control of my driver off the tee and then it felt like it was just pretty simple golf."
Founders Cup winner Yealimi Noh shot two under par in Pattaya to lie off the pace in 23rd as the tour began an Asian swing that also takes in Singapore next week and China from March 6th to 9th.
The LPGA is introducing a new pace of play policy after the Asian run, starting at the Ford Championship from March 27th to 30th in Chandler, Arizona.