BANGKOK (AFP) - Lexi Thompson secured a comfortable victory at the Honda LGPA tournament in Thailand on Sunday, a move that will push her past Stacy Lewis to become the top-ranked American female golfer.

The 21-year-old, notching up her seventh career win, hardly broke a sweat as she ended the final day with a four-under 68 and a total of 268, an unassailable six strokes ahead of runner-up Chun In Gee from South Korea.

Securing a US$240,000 cheque, she became not only the first American to win at the Chonburi course but also the youngest winner there, besting Tseng Ya-ni who triumphed in 2011 at the age of 22.

Thompson began the tournament with a blistering eight-under 64 before suffering a par-72 wobble on day two. But she recovered by day three, shooting another magnificent 64 before breezing through the final day.

The win will put her ahead of Lewis, who had held the No. 1 American spot for the last 195 weeks.

Reigning US Women's Open champion Chun put on a brave fight in the final round with a birdie on the first three holes, but victory slipped away with four later bogeys and a double bogey.

Fellow American Jessica Korda struck the best final round of the tournament with 64, enough to put her in joint third alongside last year's tournament winner Amy Yang of South Korea with a total of 275.