The 20-year-old Natthakritta, who only turned professional in November, carded four birdies and two bogeys in windy conditions at Hoakalei Country Club to take a one-shot lead with Sung into the final two rounds.
"Today my round was not really good, but okay," Natthakritta said. "But total result is good."
Sung shot a second consecutive four-under-par 68 to reach the halfway stage at eight under for the tournament alongside Natthakritta.
England's Georgia Hall and Sweden's Linnea Strom are tied for third on seven under, one shot off the leaders.
Hall was the day's biggest mover with a flawless six-under-par 66 that included six birdies and 12 pars.
Hall is hoping she can build on consecutive runner-up finishes in recent tournaments with a victory in Hawaii.
The 27-year-old was a runner-up at the Los Angeles Open earlier this month and was pipped in a playoff for the Drive On Championship crown in March.
"It's just hard work over the past year and a half," Hall said of her hot streak.
"To be honest it's all coming together and I feel very mentally confident and calm when I'm out there. Very patient as well."
Australia's Grace Kim, Japan's Nasa Hataoka and Christina Kim of the United States share fifth place on six under.
Sweden's Frida Kinhult, who had shared the first-round lead on Wednesday after a 66, tumbled down the leaderboard after a four-over-par 76 that left her six off the pace.