Munoz highlighted her round by holing from 102 yards for an eagle at the par four sixth as she also carded eight birdies and a bogey.
The Spaniard ensured a memorable round with four birdies over the last five holes, but it was still only enough for a tie at the top with Miyazato on eight under.
Japan's Miyazato shot a bogey-free 65, including back-to-back birdies at 17 and 18.
The duo hold a two-stroke lead over the chasing pack, headed by Jimin Kang and Cristie Kerr on six under.
Kang shot a six-under 66 today [Friday] while Kerr signed for a 68.
World number one Yani Tseng could only manage a 72 to fall back into a tie for 12th on three under.