Port recorded their fifth straight triumph in Thai League 1 on Sunday night to reclaim their four-point lead over defending champions Buriram United.
Port put on a dominant display at Chainat's Khao Plong Stadium for a 3-0 victory, which helped them avenge a 2-0 loss at the same venue last year.
Dragan Boskovic had a brace for the leaders and Nurul Sriyankem celebrated his return from injury by adding the third late in the match.
Port now have 23 points to their credit, four more than the Thunder Castle, who have a game in hand.
After Boskovic put the visitors ahead in the 28th minute, Chainat had a chance to pull level but Ricado Santos' shot flew over the bar.
The home team were denied another opportunity when a Chatchai Kumphaya drive was saved by Port goalkeeper Watchara Buathong.
Boskovic struck again just a minute into the second half to double Port's advantage and Nurul scored with a nifty chip over Chainat goalkeeper Teerat Nakchumnarn from outside the box 12 minutes from time.
Bangkok United's shrinking hopes for this year's title suffered another blow as they were held 1-1 by hosts Chiangmai last night.
Nelson Bonilla gave Bangkok United the lead in the 16th minute but two minutes later, Chiangmai equalised through Eliandro dos Santos to share one point each.
There was more in store for the capital side as PT Prachuap leapfrogged them into fifth place after a 2-1 home victory over Suphanburi in a match that saw three players red-carded, two of them from the visiting team.
Also on Sunday night, Ratchaburi and Sukhothai played to a 2-2 draw on a pitch left flooded by a heavy rain.
On Saturday night, Chiang Rai United netted a last-gasp penalty to edge visiting PTT Rayong 1-0.
Rayong's Chinawat Wongchai committed a handball in the box and Bill Rosimar converted the spot kick for the Beetles.