In both of her rounds to date she has mixed three birdies with a bogey, and has hit 11 of 14 fairways and required 31 putts in each round.
Putting has been Ko's crux in 2017, after a 2016 campaign where she ranked first on the LPGA Tour in putts per green in regulation, and putting average. To date in the new season, the 19-year-old ranks 41st in putts per green in regulation, and 87th in putting average.
Norway's Suzann Pettersen leads the tournament at seven-under, followed by a slew of players at six-under, including seven-time major winner Inbee Park and American Michelle Wie.
Ko's closest rival, Thailand's Ariya Jutanugarn, is alongside her at four-under after also carding consecutive rounds of 70. Jutanugarn can snap Ko's 75-week reign as world number one this weekend with a victory, assuming Ko finishes in sixth place or below.
Strong rounds tomorrow could put the pair in a similar position to last year, where they battled down the stretch in an absorbing finish. Ko came out on top at 12-under to win her second major championship, after Jutanugarn bogied the last three holes to lose what was a two shot lead with just three holes to play.
56 players are still left to finish their second round tomorrow morning before the third round gets underway.
On the PGA Tour, Danny Lee is in a share of 11th through two rounds of the Shell Houston Open. Lee shot two nice rounds of 69 to sit at six-under par, 10 shots back of runaway leader Sung Kang.