JAKARTA, 7 Dec 2014 (AFP) - The three-time Major winner held a four-shot lead at the start of the final round but stumbled at the turn in 37 as Thanyakon birdied three in his opening five holes to lead by two shots after nine holes.
Harrington clawed his way back up after a two-hour rain suspension at the Damai Indah Golf-PIK Course near Jakarta to tie with Thanyakon heading into the 18th hole.
Under strong pressure to win his first Asian Tour title, the 24-year-old Thai hit his second shot into the water for a double bogey.
Harrington sealed the win with a 15-foot par putt after taking a penalty drop when his tee shot landed on the edge of the hazard.
"Winning is a good habit to have. It gives you a lot of confidence and I need that confidence," said Harrington in comments supplied by the organisers.
He signed for an even-par 71 and a winning total of 16-under-par 268 to win $135,000.
"I didn’t start too well but I came through and I got the win. This win brings a lot of confidence to my game and hopefully it will show up next year," he added.
A dejected Thanyakon, who posted a 69, had struggled to hold his nerve on the 18th hole in the $750,000 Asian Tour event.
"It was a great final round from me but I was under so much pressure on the last hole. I'm still happy with how I played," he said.
David Lipsky of the United States extended his Order of Merit lead by slightly over $133,518 after finishing sixth. Merit runner-up Anirban Lahiri of India stumbled to a tied 15th place after closing with a triple bogey.
Leading scores after round 4 of the Bank BRI Indonesia Open played at the par-71, 7,100-yard Damai Indah GC course (a- denotes amateur):
268 - Padraig HARRINGTON (IRL) 64-66-67-71
270 - Thanyakon KHRONGPHA (THA) 67-63-71-69
271 - Nathan HOLMAN (AUS) 69-63-70-69
272 - Quincy QUEK (SIN) 71-67-68-66
274 - Sattaya SUPUPRAMAI (THA) 73-67-67-67
275 - David LIPSKY (USA) 72-68-66-69
276 - Jake HIGGINBOTTOM (AUS) 71-69-72-64, Terry PILKADARIS (AUS) 69-70-68-69, Jyoti RANDHAWA (IND) 71-68-71-66, Sam BRAZEL (AUS) 68-68-69-71, Scott HEND (AUS) 69-64-75-68
277 - Berry HENSON (USA) 72-69-70-66, WANG Jeung-hun (KOR) 70-66-76-65, Jarin TODD (USA) 69-64-74-70