SCG Muang Thong United, one of the biggest spenders in the pre-season window, suffered a confidence-shattering blow as they lost 1-0 to PT Prachuap at home in their first Thai League 1 game of the season on Sunday night.
True Bangkok United, also hoping to be among the title challengers this year, dropped points as well following a 1-1 draw at Sukhothai.
In the last match of the opening weekend, nine-man Port escaped with a goalless draw at Chiang Rai United.
The only consolation both the Kirins and Bangkok United can draw is that defending champions Buriram United were also held to a 2-2 draw by Chonburi on Saturday night.
Vietnam goalkeeper Dang Van Lam made a losing debut after conceding the only goal of the match, scored from the spot by Caion de Souza in the 27th minute.
Prachuap were awarded the penalty after Muang Thong United defender Supanan Burirat scythed down Matheus Alves inside the box. The opportunity proved enough for the visitors to record their third straight victory over the Kirins.
Muang Thong United started with four specialist forwards -- Thailand international Teerasil Dangda and Adisak Kraisorn, Myanmar striker Aung Thu and Brazilian Heberty Fernandes -- but still managed to miss a host of scoring chances.
Prachuap goalkeeper Natthapong Kajornmalee was also up to the task, making several good saves, to ensure that his team kept their 100 percent record against the Kirins intact.
At Thung Thalay Luang Stadium, Bangkok United gained the upper hand at the half-hour mark through Thailand international Tristan Do.
Sukhothai denied the capital side a winning start to the season with Kongtateechai Boonma equalising in the 76th minute.
In the other game last night, Chiang Rai and nine-man Port drew 0-0.