American Christina Kim, seeking her first LPGA title since 2005, fired a seven-under-par 65 to grab a one-stroke lead after Thursday's first round of the Lorena Ochoa Invitational.
The 30-year-old US standout took a one-stroke lead over Spain's Azahara Munoz with New Zealand teen Lydia Ko, this season's LPGA Rookie of the Year, alone in third on 68.
Kim's bogey-free round included eagles at the par-five second and 17th holes in Mexico City.
Her only two LPGA titles came at the 2004 Longs Drugs Challenge and 2005 Tournament of Champions. She won a Ladies European Tour event, the Sicilian Ladies Italian Open, in 2011.
A closing bogey cost Munoz a share of the lead.
The Spaniard opened with birdies at the first and third holes, took a bogey at the fourth but closed the front nine with a birdie. She ran off three birdies in five holes starting at the par-4 12th and eagled the 17th.
Ko, a 17-year-old South Korean-born Kiwi, has already clinched top newcomer honors with two events remaining this season.
She has won twice this year after two Canadian Women's Open wins as an amateur pushed the LPGA to allow her onto the tour a year ahead of the usual minimum age of 18.
Ko opened with a bogey but answered with back-to-back birdies on the next two holes and again at the sixth and seventh. She took another bogey at the par-5 11th and closed with back-to-back birdies to stand alone in third.
Thailand's Pornanong Phatlum and Americans Lizette Salas and Gerina Piller shared fourth on 69.
World number two Park In-Bee of South Korea, trying to overtake US rival Stacy Lewis for the top spot in the rankings, was in a pack of seven on 70 that included defending champion Lexi Thompson.
Lewis was on 76 and without a birdie in her opening round.