Pak, the 2002 champion, posted a 2-under 70 to move into a tie for second place. She stands at 5-under-par 139 alongside Catriona Matthew, Grace Park and Laura Davies.
Ahn got off to a fast start on Saturday. She rolled in a birdie at the first and followed with another at the third to climb to minus-9.
The teenager however stumbled to a bogey at the par-4 fourth. Ahn settled down from there as she ran off nine straight pars around the turn.
Ahn, a rookie on the Japan LPGA Tour, moved back to minus-9 with a birdie at the par-4 14th, but trouble loomed. She double bogeyed the 16th to drop to minus-7.
The South Korean closed out her round with a birdie at the par-5 closing hole at Nine Bridges Golf Club for the second straight day. Her birdie on the last hole gave her the new 36-hole scoring record by five strokes.
Pak, a 21-time winner on the LPGA Tour, reeled off seven consecutive pars to open her round. She moved to 4 under for the tournament with a birdie at the par-4 eighth.
The 26-year-old dropped a shot with a bogey at the 11th. She came back to birdie the par-3 13t and the three-time winner this season closed out her round with birdie at the last.
Davies, who hasn't won on the LPGA Tour since 2001, struggled out of the gate Saturday with three bogeys over her first four holes. She did manage to birdie at No. 3 in the midst of the bogeys.
She recovered from those mistakes as she ran off three straight birdies from the seventh to make the turn at 5 under. Around the turn, she carded a birdie and a bogey to maintain a share of second place.
Matthew spread two birdies and two bogeys over her 18 holes.
Park, who began the day alone in second place, struggled to two bogeys and a double bogey on the front side. She did manage a birdie at the eighth.
On the back nine, she fought back with consecutive birdies from the 12th before bogeying the 16th. She regained a share of second place with a birdie at the 18th.
Marisa Baena and Lorie Kane posted matching rounds of 3-under 69 Saturday to climb into a tie for sixth. They stand alongside Jeong Jang (68) and Soo-Yun Kang (71) at 4-under-par 140.
Lorena Ochoa recorded the best round of the day as she shot a 5-under 67. Candie Kung, Jung Yeon Lee and Rachel Teske are tied one stroke further back at minus-2.
Michelle Wie's struggles continued on Saturday. She posted a 6-over 78, seven shots better than her opening-round 85, and stands in last place at 19-over-par 163.