The tournament is hosted by the former No. 1-ranked female golfer in the world, who retired three years ago at the peak of her career after winning 27 times on the LPGA Tour.
Phatlum, seeking her first win on the tour, opened with three straight birdies at the Guadalajara Country Club and suffered a lone bogey at the par-4 14th.
Michelle Wie and Jenny Shin both shot a 69 to tie for sixth while Suzanne Pettersen was among those at 70.
Defending champion Cristie Kerr struggled to a 77 and was in last place in the 36-player field.
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