Despite bogeying two of her last four holes, Pettersen shot a 3-under 69 to finish 54 holes at 12-under 204 at Guadalajara Country Club in Mexico.
Paula Cremer, who led after Friday's second round, stumbled to an even-par 72 and found herself in sixth place at minus-10.
Tourney namesake Lorena Ochoa registered a 3-under 69, good for a 24th-place tie at minus-2.
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