The tournament is being played on the river course at the Kingsmill Resort & Spa in Williamsburg, Va.
McKay's opening round included four birdies on both the front and back nine holes and her total equaled a single-round course record.
"I feel great after shooting below my age on both 9s," McKay, 33, told the media. "I feel really good. Maybe that should be my goal for the year."
She leads both Annika Sorenstam and Sun Young Moo by one-stroke.
"It's important to score well here," said Sorenstam, who has won twice this season.
Lorena Ochoa, the top-ranked women's player in the world, shares fourth place with Diana D'Alessio at 6-under 65 while six golfers share sixth place at 5-under 66.
"I was surprised to see the low scores because the course is playing tough," Ochoa added.