The 24-year-old picked up eight birdies in a stunning round despite the windy conditions in Kohler, Wisconsin, with a bogey at the par-three 13th the only blot on her copybook as she moved to eight under.
She will head into the final round six strokes ahead of compatriot Amy Yang, who carded a 69 on Saturday, and seven clear of Germany's Sandra Gal (71), American Lexi Thompson (72) and Japan's Mika Miyazato (73).
"I will not change anything," Choi said. "I will just play my game, and I'm very excited, and this tournament is very special to all the Korean players."
Choi will be looking to emulate fellow South Korean Se Ri Pak, who won this event on this course in 1998.
"Fourteen years ago Se Ri Pak won the US Open, and I remember that feeling," Choi added. "This is a very special golf course and I'm very honoured. Se Ri Pak inspired all of the Korean players."
Midway leader Suzann Pettersen of Norway slipped to one over - nine shots off the pace - after a disappointing 78 on Saturday. American Michelle Wie, who surged into contention on Friday with a 66, also posted 78 and was two over.
Watch: Na Yeon Choi breaks away with a 65.. More Videos