Agnelo Colaco struck in the second half as Vasco SC stunned Mohun Bagan 1-0 to throw open Group D in the 30th Federation Cup at the floodlit Vidyasagar Krirnagan in Barasat on Sunday.
In the earlier match of the day, a Michael Tayo goal in the dying minutes enabled Air India hold qualifiers Oil India Limited (OIL) 1-1.
The two surprising results made it a group of death with leaders Air India (4 points), Mohun Bagan (three) and Vasco (three) having a chance to make the final-four cut.
In the last set of Group D matches, Vasco will take on OIL, already out of contention, while Bagan will face Air India on Tuesday to determine the semi-finalist.
With Sunday's win, Vasco thus stopped the Bagan juggernaut, snapping the local favourite's seven-match unbeaten streak combining both the I-League and the ongoing Federation Cup.
The day simply did not belong to the Kolkata giants, and with Bhaichung Bhutia declared off-side twice in both the halves, they simply could not set the ball rolling.
The first-half saw lapses in Bagan's mid-field and though they still had some chances, P C Lalawmpuia, Ishfaq Ahmed and Marcos Pereira were not on target.
A desperate Bagan coach Karim Bencherifa replaced Lalawmpuia with the injured Barreto in the 56th minute but the Brazilian too could not change the complexion of the match.
Mohun Bagan could have averted the defeat but for Habibur Rahman Mondal, who deflected a Bhaichung header inline with the net only to be declared off-side in the 68th minute.
From a Pereira freekick which hit the cross, Baichung took control from the rebound with a rising header but the shot was unnecessarily deflected by Mondal as Bagan failed to score.
Eleven minutes later, Colaco stunned Bagan with a powerful shot which beat goalie Sangram Mukherjee.
In the four minutes of stoppage time, Bagan had a chace to equalise but Baichung, having proved unlucky twice, failed to connect it properly as his side went down to Vasco, who are languishing at the bottom of I-League table.