In scorching heat in Evian-les-Bains, France, the 27-year-old Texan downed seven birdies in a row from the fifth and picked up two more at the17th and 18th to tie Helen Alfredsson's 2008 course record.
Lewis's superb career-low round also equalled the Ladies European Tour record for the most consecutive birdies, held by five other players.
She told www.ladieseuropeantour.com: "It was just one of those days where you just get in this rhythm where you're just hitting golf shots and not thinking about a score or trying to make birdie."
Hee Young Park, who earned her maiden victory on the LPGA last year, is two strokes back at seven under par, while fellow South Korean Ilhee Lee is a shot further behind on 66.
Watch: Lewis leads after record round... More Videos