Ewing extends Women's British Open lead to five shots
The 30-year-old led by one stroke after an opening 68 and a seven-birdie round put her on 10 under par and in control after the early starters had completed their second rounds at the last major of the year.
Ewing had a magical run of four birdies in a row from the sixth and her only mistake came at the tough closing hole where she drove into the rough and took a bogey five.
"I was definitely in the zone when I had that string of birdies," said Ewing. “But I just tried to stay in the here and now.
"It would be huge to become a major champion, it's definitely something I've circled to accomplish in my career. But now I need a good rest and get ready for tomorrow."
Lilia Vu, winner of the first major of the season at the Chevron Championship, enhanced her hopes with a second-round 68 to reach four under and US Women's Open champion Allisen Corpuz was one further back after a 71.
American Andrea Lee (68) and Japan's Minami Katsu (69) were Ewing's closest challengers in a share of second spot on five under par.
But defending champion Ashleigh Buhai shot 73 for a three-over total and was likely to miss the cut.