With this win under his belt, Thepchaiya now has a second shot at the championship.
In the semi-final round on Friday, Thepchaiya and Zhou went head-to-head in the 7-frame game, with the 37-year-old Thai star pulling through narrowly.
Thepchaiya was last crowned champion at the 2015 Six Reds World Championships held in Bangkok.
This year’s tournament, being held in Pathum Thani, kicked off on Monday and will wrap up on Saturday with Thepchaiya facing China’s Ding Junhui.
Ding beat England’s Tom Ford 7-4 on Friday to get to the finals.
The winner will grab the Royal Cup presented by His Majesty King Vajiralongkorn along with a 4.2-million-baht prize from the total winnings of 11 million baht.
As many as 32 players took part in the non-ranking event, including the four who pulled through qualifiers to compete in the final stages.