The 2011 Masters champion hit seven birdies in steamy conditions at the Amata Spring course an hour outside Bangkok and hailed his fitness after a season dogged by injury.
"I'm just playing injury-free... that's allowing me to swing the club much better," he said, explaining his strong start to the Asian Tour event in which he came second last year to a rampant Westwood.
His form bodes well after a season where he has notched just two top 10 PGA Tour finishes, but the South African refused to get carried away with three days of golf in searing temperatures ahead.
"I played really well, I didn't miss many fairways... but you're not going to win after the first round, although you sure can lose it."
He took a one-shot clubhouse lead from home star Thitiphun Chuayprakong, with Spaniard Javi Colomo one more behind at five under.
Masters champion Bubba Watson, who had promised to showcase some of his famous buccaneering "Bubba Golf" in Thailand, carded a mixed round of four under.
It included a stirring run on the back nine of birdie, eagle, then birdie, which was undone by three bogeys in an error-strewn final six holes.
"It was a solid round but I made a few mistakes," said the lefthanded American.
"All of these guys are good players, it's the first day... it's going to be hot and we're going to have to stay focused," he said.
In a star-studded field American world number 25 Hunter Mahan was two under with five played, while defending champion Westwood -- the highest ranked player at the event -- had just started his round.
Westwood cruised to a seven-shot win at the inaugural event last year on the back of an opening round 12-under-par 60 -- narrowly missing out on a magical 59, which has never been shot on the Asian Tour.