Port's title aspirations were thrown off the track by a shock 3-1 defeat at Nakhon Ratchasima as they fell 10 points behind Thai League 1 leaders Buriram United.
Port were heavy favourites to claim three points from the trip but were never able to recover from a first-half brace by Kwame Karikari.
The Ghanaian striker first converted a penalty after 27 minutes and was on target again four minutes later to deal another blow to the visitors' confidence.
Dennis Villanueva grabbed the third for the home team an hour into the match.
Port were able reduce the deficit to 3-1 through a 77th-minute Nurul Sriyankem strike, but the chances for a recovery by the Klongtoey-based team were effectively over by then.
In another game, Ratchaburi claimed a 2-0 home victory over PT Prachuap.
In Saturday night's late game, hosts Khon Kaen United rallied to salvage a 1-1 draw with Police Tero.
Jenphop Phokhee put the Fire Dragons ahead in the eighth minute but the hosts were handed the leveller by Thailand U23 striker Jakkit Phalapon 12 minutes later.
Police were reduced to 10 men on the pitch when Kanokpon Buspakom was sent off in the 65th minute for a foul on a Khon Kaen player.
The hosts failed to press home their advantage and Police ended the match with a point in their pocket.
