Port's slump continued when they lost 2-1 at bitter rivals SCG Muang Thong United in Thai League 1 on Tuesday night.
In-form forward Sardor Mirzayev of Uzbekistan gave Muang Thong the lead after 51 minutes at SCG Stadium.
Spain's Sergio Suarez equalised for the visitors 10 minutes later only for Brazilian Willian Popp to scored the winner in the 67th minute.
Second-placed Port remain 16 points behind leaders BG Pathum United who should have few problems when they host bottom club Rayong that have suffered five successive losses tomorrow.
In an entertaining game, Port's Ko Seul-Ki hit the woodwork in the 34th minute.
Muang Thong started strongly in the second half and deservedly went ahead thanks to Mirzayev's volley.
After Suarez cancelled out the lead, Ko's handball gave the Kirins a penalty and Popp made no mistake from the spot kick.
The Lions have now gone five games without a win with four defeats.
In last night's other game, Trat lost 2-0 to fellow relegation contenders Suphanburi who got two goals in the final 10 minutes through Herlison Caion and Leandro Assumpcao.
Meanwhile, in-form former champion Buriram United host holders Chiang Rai on Wednesday.
Third-placed Buriram have lost only once -- against league leaders Pathum -- in their last 10 games in all competitions.
Sixth-ranked Chiang Rai won for the first time in their last four league matches when they edged lowly Rayong 3-2 at the weekend.
Spectators return
Fans will be allowed at most stadiums in Thai league matches from this weekend, officials said.
The move came after the government's Centre for Covid-19 Situation Administration (CCSA) eased its Covid-19 restrictions, said Korrawee Pritsananantakul, acting CEO of Thai League Co.
Each stadium can welcome spectators up to 50 percent of its capacity with strict Covid-19 protocols, he said.
However, fans are still prohibited from entering stadiums in high-risk provinces, Korrawee said.