Former Thai League 1 champions Buriram United on Sunday suffered a humiliating loss at home, going down 3-1 to struggling Samut Prakan City.
Marko Scepovic brought Thunder Castle back into the game by cancelling out a 25th-minute goal by Samut Prakan's Jaroensak Wonggorn with a late first-half header.
However, the visitors took the northeastern giants by surprise again as Chakkit Laptrakul (53rd minute) and Barros Tardeli (71st) handed Samut Prakan a 3-1 advantage.
Despite waves of attacks unleashed by Buriram, Samut Prakan were able to see out the remaining 20-plus minutes and claim three points from the match.
Also last night, hosts PT Prachuap got the better of Trat 4-2 in an entertaining battle while Bangkok United were held to a 1-1 draw at home by Police Tero.
Chiang Rai confident
Chiang Rai coach Alongkorn Thongum has challenged his men to prove their victory over FC Seoul was no fluke by reproducing the same class against Melbourne Victory on Monday.
The Beetles take on the Australian club in an AFC Champions League Group E battle in Doha tonight.
Chiang Rai were routed 5-0 by FC Seoul in their first outing but they avenged it with a 2-1 upset on Friday.
The victory kept Chiang Rai's hopes of making it to the knockout stages of the tournament alive.
FC Seoul (six points), Melbourne Victory (three) and Chiang Rai United (three) are all in contention for a second round berth. Beijing lead the pool with 12 points from four games.
Alongkorn admitted that the game against Melbourne Victory would be "another tough match for us. We will have to prove ourselves once again. We will play without three suspended players but I am hoping that their replacements will rise to the occasion."
Chiang Rai goalkeeper Apirak Worawong added: "Now that we have won our first ever match in the ACL, our confidence has improved. There is a thirst to play and produce good results."
The match will kick off at 8pm (Thai time) and will be live on Fox Sports, Bugaboo TV.