Tseng, the world's top-ranked woman golfer and the winner of eight tournaments (five on the LPGA Tour) this year, shot a 7-under 65 at the Sky 72 Golf Club in Incheon. Chella Choi is in second at 6-under while defending champion Choi Na-yeon and Sandra Gal are tied for third at 5-under.
The Chois, apparently unrelated, are part of a large South Korean contingent high on the leaderboard. There are 14 South Koreans among the top 23 after Friday's first-round play.
Tseng, from Taiwan, started slowly with bogeys at Nos. 1 and 3 only partially offset by a birdie at the second hole. She began a charge with three consecutive birdies from 5-7 and made the turn at 2-under.
Tseng started the second nine with another three straight birdies. She added another birdie at 15 but gave that stroke back with a bogey at 16 that put her one stroke behind Chella Choi. Closing birdies at 17 and 18 left her alone in first.
Chella Choi had an eagle, five birdies and a bogey in shooting 66. Choi Na-yeon was bogey free. Gal had a bogey at the par-5 fifth hole against six birdies.