The South Korean shot a second straight five-under-par 67 at Sentosa Golf Club to lead by a single stroke at 10-under 134.
Playing just her second tournament after a six-month break to recover from a thumb injury, the South Korean shot a second straight five-under-par 67 at Sentosa Golf Club to lead by a single stroke at 10-under 134.
Three players -- American Michelle Wie (69), South Korean Hur Mi-Jung (67) and Thailand's Ariya Jutanugarn (68) -- were locked in a three-way tie for second at nine-under.
Norway's Suzann Pettersen (67) and South Korean Park Sung-Hyun (68) were a further shot back on a packed leaderboard.
Park had gone 29 holes without dropping a shot at the par-72 New Tanjong course when she missed the green with her approach on the 12th hole and then fluffed her chip, taking a double bogey six.
But she recovered from her mistake with a birdie at the 14th, and then calmly drained a five-foot putt at the final hole for her seventh birdie of the day to seize the halfway lead.
"It was just really unlucky. I had a really good drive down the middle and I had mud on the ball," Park said.
"I was on the right side and I knew it was going to go left, and I aimed right but it went 30 yards left. I hit a good shot but it's that unlucky bounce. Making double-bogey was a mistake.
"I didn't want to ruin the day with one bad hole with an unlucky shot. I think it's just trying to keep the calm emotions going. Obviously there was a few birdie holes coming in, so I was trying to get a couple from there."
Wie, who led overnight after firing a course record 66 in her opening round, did not make a single bogey in her second round and briefly regained the lead when she made her fourth birdie of the day at the 13th, a sweeping par-five dogleg.
But the American, who has struggled with injuries and a loss of form since winning the US Open in 2014, was unable to pick up any more shots on the run back to the clubhouse with the ball narrowly trickling past the cup several times.
"I definitely just kept very patient out there," Wie said.
"It got really frustrating at times just seeing those putts just slip out. I just kept it steady, and made a good up-and-down on the last hole which I'm really proud of."
Pettersen also briefly found herself in front after making seven birdies, including three in a row from the 10th, only to drop two shots in her last four holes.
"I played really good golf today but I had two lousy bogeys coming in," she said.
New Zealand's world number one Lydia Ko ominously charged up the leaderboard with a 68 to finish tied for seventh with five others, including the defending champion Jang Ha-Na, at seven-under.
Ko, who has gone 11 events without winning, her longest drought in four years, capped her round with three successive birdies after making a spectacular eagle on the par-five fifth when she smashed a five-wood to within a foot of making an albatross.
"I will definitely take that shot," she said. "That was the shot of the day I think."