Wiratchant moved on to 14 under par for the tournament, a run of birdie-eagle-birdie from the fifth and another birdie on the ninth helping him complete the front nine in 31.
Wiratchant picked up further shots at the 11th and 13th and it could have been even better but for a bogey on the 15th.
Fellow Thai Chawalit Plaphol and New Zealand's defending champion Michael Hendry were the closest challengers on 11 under.
Plaphol shot the best round of the day, a bogey-free 65, while Hendry went round in 69.
Four players were in a tie for fourth on 10 under, including South Korean teenager Han Chang-won, the overnight leader, who had to settle for a 71 following a double bogey at the 18th.