South Korea's Amy Yang won the Honda LPGA Thailand 2015 golf tournament in Pattaya on March 1, 2015, finishing at 15 under-par for a total of 273
Yang struck a 3-under 69 to win by two strokes in the final round of the $1.5 million tournament at the Siam Country Club, seeing off Lewis who had led after each of the previous three rounds.
The US national tied for second place with Taipei's Tseng Yani and South Korean Lee Mi-Rim -- all finishing on 13-under.
"I don't know what just happened," Yang said after the final putt dropped, according to a statement released by the LPGA Tour organisers. "I still can't believe I did it today."
She and Lewis had been tied at the top of the leaderboard heading into number 15 when she secured a birdie after the American's double bogey.
"I just kept focussing," Yang said of her thoughts at that juncture with three holes still to go, eventually going on to secure the win.
The 25-year-old South Korean claimed her second LPGA victory with Sunday's performance.
Her previous career tournament win was the LPGA KEB HanaBank Championship in 2013.
Yang has finished in the top five in three of the last four tournaments she has played in.