Park's sixth birdie on the par-5 18th at Sentosa Golf Club left her tied with Tseng, who carded seven birdies and one bogey.
Angela Stanford was in third place, one stroke back.
''The front nine is a tougher nine,'' said Park, who had five birdies after the turn. ''I didn't play too aggressively on the front nine, but the back nine I had a lot of shots.''
Tseng also had a late charge, with birdies on four of her last seven holes to build momentum heading into the second round.
The 17-year-old Ko won her previous two tournaments.
Lexi Thompson had a hole-in-one on the par-3 14th and was tied for ninth at 3-under 69.
''I hit a low controlled 7-iron from 150,'' Thompson said of her ace. ''It was looking good the whole way, so I was just waiting for it to drop. I let go of my club. I don't even know what I did honestly.''