Miyazato finished the day at 13-under 504 for 54 holes, two strokes ahead of a group of four that includes Kraft Nabisco champion Stacy Lewis and second-round leader Mika Saiki.
Saiki carded a two-under 70 on a day when conditions were conducive to low scores. Norway's Suzann Peterson shot three-under while wearing a black arm band in memory of her countrymen killed in Friday's terrorist attacks in Oslo.
Miyazato moved into a tie for the lead with a birdie on the 10th hole. She took sole possession of first place when Angela Stanford and Lewis both ran into bogey trouble on the back nine and fell off the leader board into second place.
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