Thai League 1 pacesetters Buriram United extended their advantage over Bangkok United to five points after beating the capital side 1-0 on Sunday night.
Supachok Sarachart's 89th-minute strike was enough to seal the win for the hosts.
A poor clearance by Bangkok United defender Suphan Thongsong allowed Supachok to score the winner.
Masatada Ishii remains perfect after beginning his reign as coach of the Thunder Castle earlier this month.
Under the Japanese boss, Buriram have won five games in all competitions.
"It was an important game for both teams. I'd like to praise our players who helped each other throughout the match," Ishii said. "There was quite a big crowd at the stadium. The fans helped motivate the team."
