ATK had won the first leg encounter between the two teams 3-2 on December 10 and will now meet the winners of the second semi-final between Kerala Blasters and Delhi Dynamos in the summit clash at Kochi on December 18.
There was an ugly free for all after the final whistle with the rival players and the support staff getting into a scuffle near the touch line.
Mumbai gave the captaincy honours to star India striker Sunil Chhetri in the absence of their marquee player Diego Forlan who was suspended after picking up two yellows in the first leg held in ATK's home venue, but the star forward was found wanting in the crucial game.
The Uruguayan stalwart was also sorely missed by the hosts at a venue where he had secured a superb hat-trick in the round robin stage.
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But Chhetri muffed in a one-on-one situation with rival goalkeeper Debjit Majumder, who brought off a fine save by charging out and covering the angle. The pass provider was Romanian Lucian Goian, with a brilliant punt down the middle that put the Indian striker free to score.
ATK suffered a major blow late in the half when midfielder Robert Lalthlamuana was sent off in the 43rd minute following a double-booking, reducing the former champions down to ten men.
Just prior to this attempt, ATK got the first of four yellow cards in this half when their Spanish captain Borja Fernandez fouled Brazilian Gerson Vieira.
Winger Ruidas gave a fine square pass into the box that went waste as Juan Belencoso was found wanting.
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