Buriram, who had not suffered defeat in the league since April, were four points clear heading into the midweek fixtures after Chonburi were held to a 1-1 draw at Suphanburi at the weekend.
However, the race for the title is now well and truly back in the balance - Sutthinan Pukhom scoring in the 89th minute to seal a 1-0 win over the defending champions.
Rock-bottom Samut had registered their first win in 11 top-flight attempts at the weekend but they were brushed aside with ease by Muang Thong, who are now just one point behind second-placed Chonburi.
Police United headed into the midweek round having drawn each of their last five matches.
However, a sequence of 10 matches without a win was brought to an end at Singhtarua thanks to a 62nd-minute strike from Michael Murcy.
Elsewhere, Chiangrai United's winless league run stretched to five matches as they were defeated 3-1 by Army United, Chainat Hornbill and Ratchaburi drew 2-2, and a goal apiece in the opening six minutes saw Osotspa and Bangkok Glass share the spoils 1-1.
