Miyazato made three birdies in an inspired four-hole run on the back nine as she shot her second successive five-under par score after having opened up with a tournament-record 64.
The Japanese star sits at the top of the leaderboard with a 19-under 197, ahead of Stacy Lewis, who was penalised two strokes after her round, and Jee Young Lee.
"It was a pretty solid round," said Miyazato, seeking her 10th LPGA Tour victory. "I didn't hit any bad shots and I had a few chances, a few birdie chances out there, but I couldn't make it. But still toward the end I started to feel more comfortable with my putting."
Lewis was penalised after it was discovered that her caddie, Travis Wilson, had tested the sand trap before she had played out of a bunker on the par-four 16th. It saw her 66 turn into a 68.
"I didn't see Travis do anything when we were playing. I wasn't looking at his feet, obviously," Lewis said. "So, we looked at the video and you have to slow it down, zoom in on his foot. He walks into the bunker, he kind of pushes, he kind of bounces his knees a little bit and his foot turns."
Coming off a victory two weeks ago in Singapore, Lewis could jump to number one in the world rankings with a victory in Phoenix.