BG Pathum United handed Port, who were reduced to 10 men, their first loss of the Thai League 1 season with a 4-2 home win on Wednesday night.
Singapore defender Irfan Fandi opened the scoring for the Rabbits in the 18th minute before Port's Tanaboon Kesarat was sent off in the 36th minute.
Irfan's brother Ikhsan gave BG a 2-0 lead after 42 minutes only for Teerasak Poeiphimai to cut the deficit to 2-1 just before the interval.
Teerasil Dangda, who replaced Ikhsan in the second half, made it 3-1 in the 67th minute, and Paulo Conrado scored the team's fourth goal in the 80th minute.
William Weidersjoe scored a consolation goal for Port in the dying moments.
Pathum, last season's runners-up, and big-spending Port both have seven points after four matches with each having a game in hand.
It was Pathum's second win in a row after a slow start to the season.
On the other hand, it was new Port coach Scott Cooper's first big test after matches against Khon Kaen, Prachuap and Lamphun.
They both next play on Sunday when BG travel to Prachuap and Port visit Nongbua.
Bangkok United and Ratchaburi, who both have played five games, are joint leaders on 13 points.