McKay was seven shots clear at one stage

The 28-year-old, bidding to become the first Scot to win a women's Major, added a one-under-par 70 to her opening 66 to finish six-under on 136.

And she built up a comfortable halfway lead over Americans Hilary Lunke, Angela Stanford and Juli Inkster.

But McKay's position would have been even stronger without a triple-bogey seven taken at her last hole after a run-in with a tricky bunker.

"I've never felt so pleased or relieved to hole a putt for a seven," she said afterwards.

Tomorrow, I'm going to concentrate on making good swings Mhairi McKay

"I actually didn't really feel I played the hole too badly.

"I was really unlucky with the lie in the bunker, it was plugged in the face and I couldn't really do anything with it."

But McKay was keen to look on the positive side.

"I'm still in the lead and, hopefully, I won't have any more high numbers.

"Tomorrow, I'm going to concentrate on making good swings and try to have lots of pars and birdies and just play some really smart golf."

Defending champion Inkster was delighted to be in the frame after her round of 71.

"It's mentally draining. Physically, I feel fine but you get so tired having to play 36 holes of cautious golf."