Lewis, who has one professional title to his credit, turned in a bogey-free, 9-under-par 63. He won the Portugal Masters in 2011 in just his third professional event but hasn't had a Top 10 finish since.
He's two strokes up on Joost Luiten, who started his round with four consecutive birdies and ended up at 7-under 65. Simon Dyson, Alexander Levy, Richard McEvoy and Graeme Storm are tied for third at 6-under.
Lewis began play on the second nine of the Diamond Country Club course with back-to-back birdies and had a run of three more over Nos. 15-17. After four consecutive pars, he had two birdies in the span of 4-6 before wrapping up the day with birdies at Nos. 8 and 9.