Pak and Delasin were two of only five total players to break par at the Hiddenbrooke Golf Course in Vallejo, Calif.
Cool temperatures and breezy conditions over the 6,359-yard layout made scoring quite difficult.
Behind Pak and Delasin, only Sophie Gustafson, Emilee Klein and Kelly Robbins were in the red for the day, with scores of 1-under 71.
All other 15 golfers in this elite field of 20 sat at 72 or above on Thursday.
Two-time major champion for the year Karrie Webb was alone in sixth, following a 72, while leading money winner for 2001 Annika Sorenstam shot a 1-over 73.
Meanwhile, defending champion Juli Inkster carded an opening round of 74.
Pak actually had it to 3-under for the day following consecutive birdies at Nos. 15 and 16, but let one slip when she carded a six at the closing par-5 18th.
Nevertheless, she remains on top form following her runaway victory at last weeks AFLAC Champions in Mobile, Ala.
The native of South Korea is searching for her sixth victory of the year this week. Should she obtain it, she would equal Sorenstams total on the season.
In her career, Pak has won 13 times on the LPGA Tour, including three majors, her most recent coming at this years Women's British Open.
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