The defending champion is two shots off the pace at six under, behind American Anya Alvarez, who was the day's big mover after she signed for a six-under 66 to move to eight under for the tournament.
Alvarez made six birdies during her bogey-free round and looms as the clear challenger to Ko's title defence tomorrow.
Ko, who played with a huge gallery following her throughout the day, enjoyed warm conditions in the afternoon and made five birdies but bogeys at the ninth and 18th proved costly.
She missed easy par putts on both occasions and it was hard to know whether it was down to the illness or not.
The 16-year-old said after her round that she was at the hospital this morning and had not eaten a square meal since yesterday afternoon.
At one point last night she considered withdrawing from the tournament but decided to press on.
There's a group of three players one stroke back from Ko in a share of third place at five under the card, including first round co-leader Seon Woo Bae.
After Ko, New Zealand No 2 Cathryn Bristow is the next best-placed Kiwi at one-under for the tournament in a tie for 16th after she carded a one-under 71 today.
New Zealand's Stacey Tate, who held an unlikely share of the lead after the first round, shot a horror nine-over 81 in her second round to plummet down the leaderboard as she missed the cut.
Ko will tee off in the final group at 11.30am tomorrow.
- Daniel Richardson of APNZ in Christchurch
