The 72-par tournament, a national championship that dates back to 1965, is being held under the Asian Tour flag for the first time since it was poached from them by regional rival OneAsia in 2010.
Young prospect Rattanon, ranked 394 in the world, needed just 20 putts to finish off the course on nine-under-par at Bangkok’s Thai Country Club under leaden monsoon skies.
Kaske, a 34-year-old Finn who lives in Bangkok and knows the course well, had a similarly roaring start, birdying a tough three-par sixth hole and making an eagle on the par-five seventh.
His and Rattanon’s 62 are jointly the lowest 18-hole score on the Asian Tour this season, although players were allowed to adjust their fairway balls throughout the day under local preferred lie rules that were in place.
Rattanon said heavy rains the day before aided local players like him.
“The course is playing soft due to the rain yesterday and it was an advantage for me,” the 21-year-old said according to quote put out by the tournament organisers at the end of day one in the $300,000 prize tournament.
“It’s easier to hit iron shots because you can attack the pins and try to hit it closer,” he added.
The top of the leaderboard was dominated by home players by the end of the first day.
Panuphol Pittayarat was two shots behind the leaders on 64, followed by Thai veteran Prayad Marksaen who was sharing fourth with India’s Gaganjeet Bhullar with 65.
Danthai Boonma and former Asian Tour number one Kiradech Aphibarnrat – both Thais – were sharing joint sixth one shot behind.
Australia’s Scott Hend was seven shots off the pace on 69 while America’s David Lipsky, the current Order of Merit leader, languished down the board after firing a disappointing 74.
The Asian Tour’s recapture of the Thailand Open was seen as a new blow to the struggling rival OneAsia Tour.
The announcement it had returned earlier this month came just weeks after the Asian Tour announced it had made a breakthrough deal to return to China, where OneAsia had previously had a foothold.
Leading Round I scores
62 - Rattanon Wannasrichan (THA), Janne Kaske (FIN)
64 - Panuphol Pittayarat (THA)
65 - Prayad Marksaen (THA), Gaganjeet Bhullar (THA)
66 - Danthai Boonma (THA), Kiradech Aphibarnrat (THA)
67 - Terry Pilkadaris (AUS), Scott Barr (AUS), Sihwan Kim (USA), Jaturon Duanphaichoom (THA), Keith Horne (RSA), Jazz Janewattananond (THA)
