The Spaniard had eight birdies to bring his three-round total to an 18-under 198.
He's being chased by Henrik Stenson, the first man to win the European Tour's Race to Dubai and the PGA Tour's FedEx Cup this season, who also fired a 65 to give himself a chance of a fourth win of the year. The Swede is tied in second with India's Anirban Lahiri, who fought his way into the final group on Sunday with a 67.
Second-round co-leader Justin Rose of England bogeyed his last hole for a 70 as he slipped five shots off the pace while Panuphol Pittayarat emerged as the leading Thai player following a 67 for a share of sixth place on 206.
After charging into contention with a second-round 65 that included a hole-in-one on Friday, the 33-year-old Garcia, ranked 19th in the world, put himself in prime position to land his first title of 2013.
"It was great to shoot 65 for the second successive day," said a relaxed Garcia, whose girlfriend is caddying for him.
"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."
Lahiri, who is fifth on the Asian Tour’s Order of Merit, sank six birdies against a lone bogey to stay within touching distance of the leader.
"I didn’t hit the ball very good but I played really sharp golf and that’s good to see," said the 26-year-old, who has three wins in Asia.
"I'm happy that I'm able to focus and get up and down when I miss it, and make some putts."
A total of eight players from the world's top-50 are in the field at the Thailand Golf Championship which is offering 38 world ranking points to the winner.