Lewis closed the front side at the Evian Masters Golf Club with five birdies and opened the second nine with two more while shooting a bogey-free 63.
She is two strokes ahead of Park Hee-young, who rebounded from a bogey at the fourth hole with six straight birdies over Nos. 5-10. Ilhee Lee, who had a string of five straight birdies, is third at 6-under 66.
Mika Miyazato and Mariajo Uribe each had six birdies and one bogey Thursday and are tied for fifth at 5-under 67.
Lewis opened the round with four pars before igniting the birdie run with 3s at the par-4 fifth and sixth holes. She added another at the par-5 seventh and closed the first nine with birdies at the par-3 No. 8 and the par-5 ninth. Birdies at 10 and 11, both par-4s, completed the run.
She posted five consecutive pars before closing with birdies at 17 and 18.