A third round of 70 was enough for Lewis to double her overnight lead, the American carding three birdies and two bogeys to finish 11 under par.
Thailand's Pornanong Phatlum returned a 69 to move into second place on nine under, three shots clear of Scotland's Vikki Laing, Italy's Diana Luna and Spain's Carlota Ciganda.
Sweden's Pernilla Lindberg, who had been one behind Lewis at halfway, struggled to a 75 to drop back to five under alongside compatriot Louis Larsson, while 50-year-old Laura Davies is another stroke back after a second consecutive 71.
Meanwhile, South African Connie Chen won a Mercedes E400 convertible as the first player to score a hole-in-one at the 15th, holing out from 160 yards with a six iron. Chen recorded a 70 to finish three over par overall.
Lewis, who was unable to match the brilliance of her 65 on Thursday, said: "It's always hard to follow up a day like I had yesterday, especially on the greens. It was the kind of day that could have been worse and it could have been better.
"But overall, I'm right where I need to be going into tomorrow. I just didn't hit it quite as close, didn't have as many good opportunities at birdie. But I still hit a ton of greens and if I can just get a couple more putts to fall early tomorrow, then I think I'll be in a good place."