DUBAI, December 7, 2013 (AFP) - Pornanong, who celebrated her 24th birthday on Wednesday, the opening round of the season-ending championship of the Ladies European Tour, made it a memorable week by closing with a six-under par 66.
Birdies on the last two holes helped her tally 15-under par 273 for the tournament, one shot better than Lewis, who could only par the last two holes.
The win was worth 75,000 euros for Pornanong, who had started the day two shots behind Lewis.
Spain's Carlota Ciganda (71), winner of the Ladies European Tour Order of Merit last year, was alone in third place at seven-under par 281, with Italy's Diane Luna (73) finishing in fourth a further two shots behind.
Feng Shanshan of China, winner last year, and British veteran Laura Davies were tied for fifth at three-under par 285.
Charley Hull, the 17-year-old English phenomenon, won the Rookie of the Year award as she finished tied for eighth at two-under par 286.
Her nearest rival, compatriot Holly Clyburn, who needed to finish much better than Hull, made a quadruple bogey on the 18th hole and slumped to 79 on Saturday.
Things seemed to be going exactly as per plans for Lewis as she made a birdie on the par-5 10th hole - her fourth of the day - to go to 15-under par.
At that stage, she enjoyed a three shot lead over Pornanong with no one else in sight.
It was then that Pornanong started playing her best golf.
On the par-5 13th, she made a 10-footer birdie putt, and then on the next hole, she hit a superb 4-iron to seven feet for another birdie.
Both players dropped shots on the par-3 15th hole and parred the 16th, and it was down to the last two.
On the 17th, Lewis hit her second shot to six feet, but Pornanong replied with hers to within three feet.
As the American was making her birdie putt, she got distracted by a camera going off in the crowd, and pulled her effort.
Pornanong made hers to draw level at 14-under par.
On the par-5 18th, Lewis hit her tee shot into the rough and went on to make a gutsy par from there, but the young Thai almost holed her third shot, before cleaning up the two-footer.
"I am so excited and happy. This win is going to give me a lot of confidence," said Pornapong, who has been playing on the LPGA Tour for the past couple of years.
"I really did not think I have to beat Stacy, or anything like that. All I wanted to do was play well. And I am a lot more experienced to handle the pressure now after playing with all these great players on the LPGA Tour.
A disappointed Lewis, for whom it was the 11th straight finish inside the top-eight since July this year, said: "It's been a really good year. It's had its highs and lows. I've played some really consistent golf.
"These last few months have kind of left a sour taste on it, just kind of losing a couple of tournaments right at the end."
"But if anything, that leaves me with stuff to work on in the off?season and just shows that there's always room to get better."
Leading scores after the final round in the 500,000 euros Dubai Ladies Masters on Saturday at the par-72 Majlis Course of Emirates Golf Club:
273 - Pornanong Phatlum (THA) 68-70-69-66
274 - Stacy Lewis (USA) 70-65-70-69
281 - Carlota Ciganda (ESP) 67-70-73-71
283 - Diana Luna (ITA) 73-68-69-73
285 - Shanshan Feng (CHN) 76-67-71-71, Laura Davies (ENG) 70-71-71-73, Louise Larsson (SWE) 72-71-68-74
286 -_ Charley Hull (ENG) 70-72-73-71, Jing Yan (Am-CHN) 75-72-68-71, Vikki Laing (SCO) 71-70-69-76
287 - Nontaya Srisawang (THA) 73-72-71-71, Pernilla Lindberg (SWE) 67-69-75-76
288 - Caroline Masson (GER) 69-72-78-69, Anne-Lise Caudal (FRA) 72-73-70, Sarah Kemp (AUS) 70-72-72-74
289 - Margherita Rijon (ITA) 73-76-72-68, Patcharajutar Kongkraphan (THA) 70-78-73-68, Klara Spilkova (CZE) 74-73-73-69, Sophie Gustafson (SWE) 73-72-71-73, Cindy Lacrosse (USA) 73-70-73-73, Gwladys Nocera (FRA) 71-73-71-74
290 - Thidapa Suwannapura (THA) 71-74-72-73