The world number three, who already won the Mobile Bay LPGA Classic and the ShopRite LPGA Classic in Alabama this year, birdied four of the first five holes before going on to shoot a bogey-free round.
“I think going into today I knew I needed to make birdies just the way the scores had been all day, and you don’t know if you’re ever going to start off like that,” Lewis said.
“You can kind of hit it good warming up but you never really know what you’re going to take to the course.
“I wasn’t happy with the way I played the back nine yesterday. Putts weren’t going in. They were going to the edge and not going in so I worked on some basics yesterday.
“I think coming back out and getting birdies on the first two holes really gave me some momentum and confidence.”
Angela Stanford sits in second, two shots adrift of Lewis, after a third round 68.
"To win early in the year and late in the year would be pretty big, because I think it's such a long year that there's so many ups and downs," said Stanford, who won the HSBC Champions in Singapore in February.
"It would be nice to have another."
Overnight leader Lexi Thompson fell five shots behind, for a share of sixth, as she struggled with a round of 74.
The 17-year-old defending champion had opened with a 63 to match the tournament record before posting a round of 70 on day two. But a double bogey on 12 as well as two more bogeys cancelling two birdies saw the American slip to a total of 10-under.
"I just struggled a little bit," Thompson said.
"I'm definitely not out of it. I shot 63 the first day, so if I have a round like that, I can be in contention. I'm just going to go out and free swing and try to make birdies. Hopefully, a few more putts drop."