The Gaurs thrashed the Kolkata side 5-1 thanks to a brace from Spanish midfielder Manuel Lanzarote and goals from Ferran Corominas, Mark Sifneos and Sergio Juste. Robbie Keane pulled one back for the visitors.
The defending ISL champions started the game on a positive note and looked in control until the 10th minute. But after Juste's goal, they slowly faded away from the match.
Speaking about the game, ATK coach Ashley Westwood said, "We had a game plan. I think we started very well. We were the better side in the first 10 minutes to be completely honest. They found it hard to get into the game where we had a few chances.
"Robbie Keane was adamant that he had a definite penalty for a big shirt pull. At 0-0 that makes a difference. Decisions can go against you which makes it hard but if you concede from a set piece, then you are low on confidence. They get a little lift as they get a goal and then it's hard to match their attack."
ATK winger Zequinha came back into the side after prolonged injury period. The Portuguese, who has arguably been the best performer for the team this season, started off the bench in this crucial tie.
"Zequinha has been out with a hamstring injury five weeks ago. So he was never in the plan to get too many minutes today (Wednesday). He went back to Portugal for some rehabilitation. He got himself fit but it was always a risk putting him on. We shouldn't have actually played him, but credit to him for having come on and done well."
It has been a nightmare of a season for the reigning champions. A team who had finished among the top four teams in the last three editions are now fighting to avoid the wooden spoon.
The team has been plagued by injury problems from the very beginning of the season. Westwood once again pointed out how injuries have killed ATK's season.
"Collectively, across the tournament, without a doubt. Robbie Keane missed the first few games. If you take out all the key foreigners from all the key sides, then you will struggle. If you look at Pune (City) - Marcelinho, Bengaluru (FC) - Miku (Fedor), Goa with (Manuel) Lanzarote and Coro (Ferran Corominas). We've also had Eugeneson (Lyngdoh), who is the best Indian in the side, missing. That's not an excuse as you have 25 players who can make a difference.
"Someone tells me we've had over 40 injuries at the club across the season or in 17 games which is alarming," said the two-time I-League winning coach.
This season has been Ashley Westwood's worst performance as a coach in India. A man who was used to success during the three seasons he was in charge of Bengaluru FC, Westwood faced failure since he took charge of the ATK side.
Under Westwood, ATK are yet to win a game. Speaking about the terrible season, the British coach said, "It's extremely frustrating. I find it emotionally hard because you want to be a winner. I was a winner as a player and a coach. I also know how to make a winning side. You need to get the players early, put your systems in place, drive fitness, nutrition and team spirit. Obviously, you have to coach and educate.
"With those many injuries as well, you're basically just keeping the seat warm and trying to get to a stage where you can make a difference from scratch. Pre-season is your key work. That's where you put all your rules and regulations, put all your disciplines and fitness regimes in place. To come in and not being able to effect that at this part of the season is the reason why you don't always get the results you're after."