Maharoof (5/42) was the wrecker-in-chief with a hat-trick in the 39th over as India collapsed for a paltry 209 in 42.3 overs.
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India were cruising at 189 for four in the 38th over when Maharoof changed the course of the match in a span of three balls, dismissing Ravindra Jadeja, Praveen Kumar and Zaheer Khan for a first ball duck.
Rohit Sharma (69 off 73), Dinesh Karthik (40) and skipper Mahendra Singh Dhoni (41) were the main contributors in the Indian innings.
Kumar Sangakkara (73) and Mahela Jayawardene (53 not out) produced 104 runs for the third wicket to steer Sri Lanka to an all-win record in the league with bonus point to boot.
With their confidence pepped up after yet another Indian conquest, Sri Lanka, who garnered 14 points from three successive wins, can seek to upstage Dhoni's men in the all-important final on Thursday.
Given the ball in the 39th over, 25-year old Maharoof trapped Jadeja right in front of the wickets off the first ball and then castled Praveen with the second.
With the crowd rooting behind him, Maharoof had Zaheer caught brilliantly by a diving Sangakkara to complete a dream hat-trick.
Maharoof became the third Sri Lankan after Chaminda Vaas (twice) and Lasith Malinga to complete the coveted feat. It was the 26th hat-trick in ODI history.
Put into bat, India were sitting pretty at 189 for four in the 38th over before Maharoof triggered a collapse with three wickets in as many balls to be all out in 42.3 overs at the Rangiri International stadium.
Maharoof trapped Ravindra Jadeja right in front of the wickets off the first ball of the 39th over. He then castled Praveen Kumar with the second.
As the motley home crowd egged him on for a hat-trick, the 25-year-old pacer had Zaheer Khan caught brilliantly by a diving wicketkeeper Kumar Sangakkara to complete the feat.
India lost four wickets in four balls as captain Mahendra Singh Dhoni (41) was run out of the last ball of the previous over bowled by Rangana Herath. Dhoni was beaten to the crease by a Chamara Kapugedara direct hit as he strove to steal a non-existent single.
Maharoof became the third Sri Lankan after Chaminda Vaas (twice) and Lasith Malinga to complete the coveted feat. It was the 26th hat-trick overall in ODI history.
Invited to bat by Sri Lanka, India were in troubled waters at 110 for four before Rohit Sharma and Dhoni raised 79 runs for the fifth wicket.
Amidst the Indian ruins Rohit showed maturity during his tempered innings, eschewing stroke play for deft placements.
Opener Gautam Gambhir (23) was the first Indian batsman to perish. Venturing to pull Angelo Mathews, the left-hander saw Suraj Randiv lunge to his right and pluck the ball out of the air at short square. An outstanding catch it was.
Hardly had India recovered from Gambhir's premature demise when Virat Kohli, batting at the pivotal number three position, nibbled at a Maharoof delivery for Sangakkara to complete a regulation catch. He made 10 runs, registering his third successive failure in the tournament.
And when Dinesh Karthik (40), making his ninth comeback in 45 ODIs, was gobbled up by Sangakkara off left-arm spinner Rangana Hearth, India were in deep trouble at 77 for three in 15.2 overs.
Suresh Raina (18) and Rohit defied the Lankan attack for a while before the former, attempting to sweep Herath, was trapped in front of the wicket by the left-arm spinner.
Had it not been for the stubborn fourth wicket stand between Rohit and Dhoni, India could have been bowled out for
a total much lesser then they eventually made.
