Pressure mounted on Chonburi coach Sasom Pobprasert as the Sharks lost their fifth game in six Thai League 1 outings Sunday night, going down to Ratchaburi 2-1 at home.
By the end of the first half, Ratchaburi had captured a two-goal advantage through strikes from Derley (32-penalty) and Safawi Rasid (45+4).
Chonburi pulled one back when Danilo Alves shattered the net with a spectacular bicycle kick in the 56th minute.
In another match Sunday night, hosts Police Tero were held to a 1-1 draw by Lampang.
Meanwhile, Buriram United coach Masatada Ishii is hoping to wrap up the title in the Thunder Castle's next game against second-ranked Bangkok United.
Ishii's men showed no mercy to Khon Kaen United in a northeastern derby on Saturday night, hammering their opponents 4-1 at home.
Lonsana Doumbouya was the star performer for the hosts as he grabbed a hat-trick by finding the net on 20, 45+8 and 79 minutes of the match. Supachai Chaided completed the rout with a fourth goal after 83 minutes.
Khon Kaen's only goal was scored by Ibson Melo at the stroke of half-hour mark.
Ishii was happy with his team's performance as defending champions Buriram moved to the verge of claiming their eighth league crown.
"We didn't play well in the first half because of the hot weather which affected both sides," he said.
"We changed the tactics after the break. We weren't perfect but Khon Kaen also committed mistakes so we were able to score four goals and win the match.
"We must now try to claim three points from our next game against Bangkok United and seal the title."
In another Saturday night game, Nakhon Ratchasima shocked former champions Chiang Rai United 3-1 at their home ground.