Meena Lee is alone in third place after a 6-under 66, while Hee-Won Han is one shot further back in fourth place after a 67.
Following back-to-back wins by Lorena Ochoa, the LPGA Tour is in Asia for the first of three consecutive tournaments on the continent.
Among the unique features of this event -- the first LPGA tournament held in Thailand -- is a true island green, where players are ferried by boat to their balls.
The 145-yard, par-3 17th at Amata Spring Country Club is Asia's only island green. Both of the co-leaders escaped with par there in the first round.
Forty-three players in a field of 60 shot par or better Friday while negotiating lakes, rivers and considerable elevation change at the challenging par-72 layout.
And although many of the top women's golfers in the world are not in the field -- including Ochoa, Karrie Webb and Annika Sorenstam -- there remains personal interest for the handful of LPGA Tour players with ties to the host country.
Some of them performed better than others.
Stacy Prammanasudh, whose father was born in Thailand, opened with an even-par 72 and is tied for 35th place.
Aree Song returned to her home country and shot a 2-over 74 to end the day tied for 48th place.
Virada Nirapathpongporn, an LPGA Tour rookie who grew up in Bangkok, opened with a 5-over 77. The second-worst score of the day left Nirapathpongporn alone in 57th place.
Young played bogey-free for her 65, while Castrale collected eight birdies and a bogey.
World No. 4 Cristie Kerr, the highest-ranked player in the field, is six shots back after opening with a 1-under 71.