Bangkok United's bid for their first-ever Thai League 1 title was thrown further off the track after a shock 1-0 defeat at struggling PT Prachuap on Sunday night.
A goal by Willen Mota on 70 minutes for the Wasps deflated the capital side's hopes for a victory.
The Angels now trail leaders Buriram United by eight points with nine games left this season.
In another game last night, Ratchaburi rallied to salvage a 2-1 home win over Suphanburi.
William Henrique guided the ball past Ratchaburi goalkeeper Kampol Pathomattakul from a narrow angle to give Suphanburi the lead in the fifth minute which Kasitdet Wechayawong erased soon after the break.
Sittichoke Kannoo grabbed the winner for the hosts after 80 minutes.
On Saturday night, Brazilian striker Diogo Luis Santo converted a last-gasp penalty to give defending champions Pathum United a 2-1 win at Chiang Rai United.
The Rabbits went ahead early when an Irfan Fandi header was deflected into the goal by a Chiang Rai defender.
The Beetles recovered from a penalty miss by Getterson dos Santos at the half-hour mark to level the score through a Kohei Sato goal early in the second half.
The hosts conceded a penalty moments before the final whistle when Shinnaphat Leeaoh was adjudged by the VAR to have a committed a handball inside the box. Diogo had no trouble planting the ball into the net.
Pathum's interim coach Surachai Jaturapattarapong said: "We had to rotate some players but our tactics remained unchanged.
"Chiang Rai were good tonight and we were lucky to get that penalty towards the end."
Chiang Rai coach Emerson Pereira said: "[I] Have no complaints because we played well in the second half.
"It's a pity that we couldn't win."