At the tournaments halfway point, the 26-year-old sits at 10-under-par 134, one shot ahead of Carin Koch (67) and two better than Grace Park (66) and Fiona Pike (68).
Laura Diaz (67) resides alone in fifth place at minus-7.
McKay began her round at the par-4 10th and picked up one lone birdie over her first nine holes to get to the 3-under mark.
But then she got it going, reeling off six straight birdies between Nos. 2 through 7 (her 11th through 16th).
One time I had eight birdies in a row as an amateur, she commented following her round. I didn't really think about it until putt seven went in, it was off the slope and that was sort of a nice extra bonus. Then on eight I probably had my easiest chance and I missed it. That's just the way it happens.'
I have been playing pretty consistently for the past six weeks, she went on to say. When I came up for this tournament, I was happy with the way my game was shaping up. I knew that if I could keep doing the things I had been doing and make a few putts, that I could shoot a low number -- and that's what I did today.
For the record, McKays 65 was the best of the day.
Koch was not far behind, however. Likewise with McKay, her round consisted of no bogies.
Yet, for as good as it was, it might even have been better.
Even though I managed to get a good score I didn't birdie either of the two par 5s that are reachable, Koch said. And that is almost like two bogies today because they were both really short. So that's the two shots I feel like I kind of gave away. I played well on the other six holes.
The cut came at even-par on Friday, which marks a new all-time low cut for the Corning Classic. Just missing that mark was defending champion Betsy King, who shot rounds of 73-72 to finish at 1-over.
Full-field scores from the Corning Classic