New Zealand's Ko in three-way tie for lead at Blue Bay LPGA
New Zealander Ko, who won the season-opening Tournament of Champions, is in a three way tie for the lead on 12-under par 204 with American pair Bailey Tardy who also shot 66 and Sarah Schmelzel (69).
"I played really well on the front nine again like I did yesterday," said world number seven Ko who holed her approach for an eagle on the par-five third.
"Not often that you're three-under through three two days straight. To be able to make a good pitch, chip-in eagle on three was definitely a good momentum booster.
"I played really solid. Didn't put myself in that many odd places, and I feel like you can do that quite easily around here. Hopefully more of that tomorrow."
Schmelzel is in good touch, having finished in a share of eighth place at the HSBC World Championship in Singapore a week ago.
"I've been having a ton of fun this week and last week being in the mix a little bit more," said the world number 95.
"A big goal going into this year was just to be in the mix more often, so it's nice the last two weeks I've gotten myself in that position, and it's going to be exciting."
Top-ranked Lilia Vu withdrew from the Blue Bay LPGA in China before the second round, opening the door for Celine Boutier to become world number one.
Boutier will become the first golfer from France, man or woman, to be ranked number one if she finishes solo third or better on Sunday at the Jian Lake Blue Bay course on Hainan Island.
But world number three Boutier has a huge amount of ground to make up in Sunday's final round to finish in the top three.
She is four-under par after 68 and eight shots off the pace in a tie for 22nd place.
Boutier, 30, who won her first major last season at the Evian Championship in her native France, was second last week at the HSBC Women's World Championship in Singapore, a shot behind winner Hannah Green of Australia.
Boutier, now in her eighth season on the LPGA Tour, finished third in her only previous appearance at Blue Bay in 2018.