Former champions Salgaocar held on gamely to a first half goal to get the better of Mohun Bagan 1-0 in a seventh round match in the National Football League in Margao, Goa, on Friday.
The victory lifted the Goa team to fifth place on the league table.
The 1998-99 champions, who controlled the midfield with more ball possession in the first half, scored the all- important goal in the 34th minute.
Climax Lawrence, from the right, gave a long pass to Felix Ibrebru, who chested the ball inside the box and unleashed a right-footer on second attempt to find the far corner of the net.
Bagan, champions in 197-98, 1999-00 and 2001-02, pressed hard in the second half and held more of the exchanges but met with stiff resistance from the rival defence, in which Bello Rasaq was outstanding.
The victory should be a morale booster for Salgaocar, who had struck a lean patch, losing three successive matches after beating defending champions East Bengal and Mohammedan Sporting.
They now have 10 points from seven matches while Bagan, who lost their second match, remain in third place with 11 points.
Salgaocar, who needed a victory badly to arrest their slump, dominated the first half as their midfield played in unison. But though they controlled play they could not split open the rival defence.
A Jatin Bist long ranger narrowly missed the Bagan goal while Felix Ibrebru, who was found clear inside the box, failed to direct his shot on target.
The Salgaocar midfield did well to check Bagan's danger man Jose Barreto, but the wily striker did well once to elude his marker and lay a long pass for Ashim Biswas inside the box. However, Biswas's attempt was deflected out by onrushing keeper Rajat Dastidar.
But the change of ends saw Bagan playing with more vigour as they pressed hard for the equaliser but found the rival defence a tough nut to crack.
The only chance the Kolkata giants got was midway in the second half when Rocus Lamre crossed from the left to unmarked Leonard D'Almeida, who headed out.
Salgaocar came off with counter moves and had two chances to score but once Jatin wasted a clear chance after he got the better of defender Harpreet Singh, and earlier keeper Prasanta Dora came out to deflect Jatin's powerful shot.
Referee S Suresh showed yellow cards to Salgaocar's Climax Lawerence and Bagan's Harpreet Singh and Mehtab Hossian.