Thompson, 18, is seeking her second tour title and is trying to break through in a year that has been mildly disappointing. She has only one top 10 finish in eight previous starts.
Jessica Korda was alone in third place at 66 while the group at 67 included Nicole Castrale and Mina Harigae.
Defending champion Stacy Lewis, ranked second in the world, settled for a 70 that left her in a tie for 33rd.
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