Sukhothai's bid to remain in Thai League 1 next season was dealt a blow when they lost 3-1 at Chiang Rai United last night.
Sukhothai's Sila Srikampang scored an own goal to give Chiang Rai the lead after 28 minutes.
Striker Bill Rosimar made it 2-0 shortly after the break and John Baggio cut the deficit to 2-1 from the penalty spot in the 56th minute.
As Sukhothai piled the pressure on the Beetles, it was the Beetles' Akarawin Sawasdee who finished them off after 83 minutes.
The Fire Bats remain fourth from last on 27 points, four ahead of Suphanburi who have a game in hand.
Bottom clubs Rayong and Trat have already been relegated.
In yesterday's other game, PT Prachuap beat Muang Thong United 1-0 with Rudolof Yanto Basna scoring the winner after 33 minutes.
In today's action, relegation strugglers Suphanburi host BG Pathum United.
Although Pathum have already won the title, the new champions still want to finish the season without a defeat to match the feat previously achieved by Buriram United and Muang Thong United.