She is in a share of second place on the individual rankings with Australian Whitney Hillier.
Tokoroa's Chantelle Cassidy shot a four-over 76 while Hamilton's Emily Perry shot the non-counting 78.
Hyo-Joo, the world No 17, has a runaway lead in the individual race on 10-under par 134 after 36 holes, as Korea streak towards their sixth consecutive Queen Sirikit Cup victory, having won 14 times over the event's 34-year history.
Korea, led by a red-hot Hyo-Joo, managed the windy but sunny conditions superbly at the Tanah Merah Country Club Garden course.
She started the day with five consecutive birdies to be on five-under par at the turn before ending her day with nine birdies and a seven-under-par
65, leaving the rest of the field trailing by several shots.
This was her second-best round ever in a tournament, having chalked up 11 birdies during an eight-under-par 64 at an age-group tournament in Jeju.
Meanwhile, Ko struggled with her putting during her round yesterday.
"I was trying to play my normal game, but I noticed I was short on a number of putts. I realised I was actually playing more conservatively compared with Wednesday," Ko said.
"On a positive note, I started well, managed to birdie the third hole, but it was all average after that."
Queen Sirikit Cup - Leading scores after two rounds:
277 Korea 142 135
292 Australia 145 147
294 Philippines 147 147
295 New Zealand 146 149 (Lydia Ko 69 73, Chantelle Cassidy 77 76, Emily Perry 79 78)
300 India 152 148
301 Indonesia 151 150, China 152 149, Malaysia 147 154
303 Thailand 157 146
305 Japan 152 153
307 Chinese Taipei 154 153
310 Hong Kong 152 158
317 Singapore 160 157