BANGKOK, December 14, 2013 (AFP) - The Spanish star surged to 18-under for the tournament with eight birdies and just one bogey.
Matching Garcia's 65 was Sweden's pre-tournament favourite Henrik Stenson who moved into joint second place along with India's Anirbarn Lahiri.
Joint overnight leader Justin Rose slipped back to fourth spot after a disappointing 70, leaving the Englishman five shots behind the leader.
Two shots further back, alone in fifth place, was France's Alexander Levy while the best score of the day came from Korea's Kim Gi-Whan whose eight-under 64 catapulted him into joint sixth spot.
"It was great to shoot 65 for the second successive day," said a relaxed Garcia. "I felt very comfortable and hit some nice shots. I was particularly pleased with my putting, especially sinking birdies on the difficult last two holes."
Despite his healthy lead he knows there is still plenty of work to do and won't be playing cautiously in Sunday's final round.
"There are five or six guys out there who can shoot a good number," he said. "I'll be as aggressive as possible. I won't be playing defensively."
Stenson looked particularly pleased after his bogey-free seven-under round. "That's the best I've played all week," he said.
"In addition to the birdies, I made some good saves which kept the momentum going, which was important."
However, he knows overhauling Garcia will not be easy. "It's going to take some special golf to catch Sergio," he said.
Lahiri was pleased to be in contention after shooting an excellent 67. "I'm very happy as I did not hit the ball that well," said the promising 26-year-old.
"I'm glad I'm within touching distance of Sergio, but I won't expect too much. I'll just do my best."
Rose admitted he had experienced a "frustrating day" missing a number of birdie putts. "I played quite well and had the opportunity to put a good round together, but didn't do it," he said.
The US Open winner said he would do his best to catch Ryder Cup teammate Garcia in the last round.
"It's my last round of the season. I'll shoot at the flags and hope the putts go in."
Leading the local challenge is Thailand's Panuphol Pittayarat in joint sixth place after shooting 67.
But it was compatriot Chinnarat Phadungsil who prompted the biggest cheers from the gallery with a hole-in-one at the fifth, the second ace of the tournament.
The ace climaxed a blistering start for the young Thai who found himself five-under after the first five holes on his way to an impressive 65 for joint ninth place.
Scores after the third round of the $1 million Thailand Golf Championship on Thursday at the Amata Spring Country Club (par 72).
198 - Sergio Garcia 68-65-65 (ESP)
202 - 70-67-65 Henrik Stenson (SWE), 71-64-67 Anirban Lahiri (IND)
203 - 65-68-70 Justin Rose (ENG)
205 - 70-65-70 Alexander Levy (FRA)
206 - 74-68-64 Kim Gi-Whan (KOR), 72-69-65 Yuki Kono (JPN), 71-68-67 Panuphol Pittayarat (THA)
207 - 74-68-65 Chinnarat Phadungsil (THA), 70-66-71 Rickie Fowler (USA)
208 - 73-69-66 D.A. Points (USA), 68-73-67, Charl Schwartzel (SAF), 72-69-67 Chapchai Nirat (THA), 66-73- 69 ,Gaganjeet Bhullar (IND), 65-74 -69 Arnond Vongvanij (THA), 72-65-71 Andrew Dodt (AUS), 68-69-71 Sujjan Singh (IND), 69-68-71 Kiradech Aphibarnrat (THA), Alex Cejka 64-71-72
209 - 72-71-66 Berry Henson (USA), 69-73-77 Marcus Fraser (AUS), 71-70-68 Wade Ormsby (AUS)
Selected scores
210 - 71-71-68 Thongchai Jaidee (THA), 74-68-68 Ryo Ishikawa (JPN)
211 - 71-72-68 Hunter Mahan (USA)
216 - 73-70-73 John Daly (USA)