Brittany Lang remained in second place at minus-7 after a 71 of her own. Seon-Hwa Lee climbed into a share of second thanks to a 5-under 67 in round two.
Rachel Hetherington, the 2003 champion, carded her second straight 3-under 69. She was joined in fourth place at 6-under-par 138 by Wendy Ward (68), Nicole Castrale (69) and Jamie Hullett (67).
Ahn opened with a 10-foot birdie on the par-4 second to move to minus-9. On the par-3 fifth at Locust Hill Country Club, she stumbled to her first bogey of the event.
The 21-year-old tripped to another bogey at the sixth when she was unable to get up and down from a bunker. After a pair of pars, Ahn recovered one of those lost strokes with a birdie at No. 9.
Ahn, who won the 2003 CJ Nine Bridges in her first tour start, birdied the par-5 11th to get back to 8 under par. She climbed to minus-9 with a 7-foot birdie putt on the par-3 15th. Ahn, however, bogeyed the 16th before closing with back-to-back pars.
'I was trying to play as well as yesterday, and I was trying to focus on my driver,' Ahn stated. 'I was trying to hit a lot of draw shots on the driver, but it didn't work out well. My drives ended up going straight, so I had a lot of balls in the rough and that was a little hard, but it ended up being a good day.'
Lang played the front nine in 1 over par as she faltered to bogeys at one, five and nine. Lang's birdies on her opening nine came on Nos. 3 and 8.
The former Duke standout got to 1 under for her round with consecutive birdies from the 11th. Lang gave another stroke back with a bogey at the par-4 16th. She atoned for that error when she got up and down for birdie on 17 to share second.
'The course played a lot easier today, but I didn't hit the ball very well, and didn't putt very well,' Lang admitted. 'I made a few putts coming in, but I made a couple of stupid mistakes and bad tee shots.'
Lee, who won the ShopRite LPGA Classic and has three runner-up finishes this year, got going with a birdie on the first and another on the third. She tripped to her lone bogey at No. 5.
The 20-year-old erased that mistake with a birdie from 8 feet on the par-3 seventh. Lee birdied the par-5 11th from 20 feet away to move to 5 under. She grabbed a share of second place thanks to birdies on 14 and 18 to close out her round.
'It was a great day. The weather was nice, not too hot, not to cold,' said Lee. 'My drives were good on the front nine. My putter was great. The back nine was not too bad, I just missed the fairway a little bit on Nos. 17 and 18. I birdied the last hole, which makes me happy.'
The cut line fell at 2-over-par 146 with 73 players moving on to the final two rounds. Among those missing out on the weekend were Ai Miyazato (147), Morgan Pressel (148), Rosie Jones (149) and Pat Hurst (150).