Thailand's Pornanong Phatlum took the first-round lead in the Lorena Ochoa Invitational, birdieing seven of the first 13 holes at Guadalajara Country Club on Thursday.
Pornanong, winless in five seasons on the US LPGA Tour, dropped a stroke on the par-4 14th hole and finished with a six-under 66.
Amy Yang was a stroke back, and fellow South Korean players Inbee Park and So Yeon Ryu were two shots back at 68 along with Sweden's Anna Nordqvist.
The top-ranked Park swept the first three major championships of the season and has three other tour wins this season. She's winless in eight starts since the US Women's Open in late June.
Second-ranked Suzann Pettersen opened with a 70.
Defending champion Cristie Kerr was last in the 36-player field at 77.
Tournament host Lorena Ochoa won 27 tour titles before retiring in 2010.
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