Ji won her fifth U.S. LPGA Tour title in her previous event last month, the season-opening Tournament of Champions in Florida.
She picked up right where she left off, in Chonburi.
After an opening birdie and a bogey on the third hole, Ji birdied nine out of the following 14 holes on the Siam Country Club Pattaya's Old Course.
In her wake were Minjee Lee of Australia, Danielle Kang of the United States, and Jenny Shin of South Korea, each with a 65.
"I felt really great. I didn't miss any fairways and missed just two greens," Ji said. "I have a lot of confidence.
Lee was bogey free as she eagled the 15th and birdied five holes on a humid day.
"I was little bit slow starting out. I finished with three birdies on the last three holes at the front nine, and got a little bit better momentum," Lee said.
A group of four, three strokes off the lead, included Americans Jennifer Song and Lizette Salas, Canadian Brooke Henderson, and Australian Katherine Kirk.
World No. 1 Ariya Jutanugarn, the 2013 runner-up, carded a 4 under 68, mixing an eagle and six birdies with two bogeys and a double bogey.
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