Manipur sealed the issue in the first-half with a brace from Lunminlen Haokip (19th and 42nd minutes) and another goal from Somishon Shirak just before the breather.
The win meant Manipur now have nine points after four games and have secured a last-four berth.
Karnataka (four points from three matches) will have to win their final game against Gujarat on Monday and hope for other results to go their way to make the last-four.
The first goal of the game came from a terrible mistake by young L Darshan’s as his loose pass was intercepted by an alert Somishon Shirak who picked out an onrushing Haokip to give Manipur the lead.
A chipped through ball from Ngulgoulal Singsit found Haokip who broke the off-side trap before deceiving Siju S to bring up his second.
Manipur struck their third from a through ball that found Shirak with a lot of space. His first shot was saved superbly by Jayanthkumar Krishnamurthy but Shirak poked home the rebound to make it 3-0 at the stroke of half-time.
In the second-half, Manipur were content to hold their shape and try to hit Karnataka on the break.
Karnataka put up a much improved display after the break but they had nothing to show for their efforts.
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