Buriram enjoyed a comfortable 3-0 win against Songkhla United but Muang Thong needed two late goals to finish off Chainat.
At I-Mobile Stadium, Philippines forward Javier Patino scored twice and Argentina's Leonel Altobelli added the other as Buriram had few problems against the visitors.
Patino opened the scoring after just seven minutes. But the home team had to wait until 14 minutes from time to get their second goal through Altobelli. The Filipino put the match beyond any doubt in the 83rd minute.
Songkha won a penalty in the dying moments but Kraikitti Inn-utane's shot was saved by Buriram goalkeeper Sivaruk Terdsungnern.
Buriram will travel China to face Shandong Luneng in the AFC Champions League in midweek.
At SCG Stadium, Muang Thong, who were eliminated in the AFC Champions League play-offs last week, were stunned by Chainat after 21 minutes when Panuwat Failai gave the visitors the lead. But Wuthichai Tathong equalised after 76 minutes and Mario Gjurovski's penalty kick with two minutes to go gave Muang Thong the three points.
At Chonburi Stadium, Chonburi thrashed Police United 4-0.
Brazilian Thiago Cunha scored two goals after 33 and 43 minutes. Anderson dos Santos, another Brazilian, and veteran midfielder Therdsak Chaiman added two more goals in the final 15 minutes.
At United Stadium, hosts Chiang Rai lost 2-0 to Bangkok Glass with Teerathep Winothai scoring twice in the first half.
At Suphanburi Stadium, Suphanburi were stunned 2-1 by Ratchaburi.
Ratchaburi raced to a two-goal lead through Heberty Ferdendes (42) and Genki Nagasato (58), and Wuthichai Asusheewa cut the deficit to 2-1 after 87 minutes.
6pm _ Si Sa Ket v Osotspa, Samut Songkhram v BEC Tero
7pm _ Air Force v TOT, Army v Bangkok United
8pm _ Singhtarua v PTT Rayong