New Zealand revived their Twenty20 World Cup campaign with an eight-wicket thrashing of India, whose own semi-final hopes suffered a jolt after their second successive defeat in the tournament on Sunday.
Put in to bat, India's rejigged top order wilted in no time and the 2007 champions crawled to 110-7 in 20 overs in the Super 12 match.
The below-par total was unlikely to test New Zealand's batting might and so it proved.
Daryl Mitchell made a breezy 49 and Williamson remained not out on 33 as New Zealand cruised to their first win of the tournament with 33 balls to spare.
"A fantastic all-round performance from us against a formidable India side," Williamson said after the victory.
"We were able to build pressure throughout and the way the openers came out really set the platform."
The Group II match featured two teams who were beaten by group leaders Pakistan in their respective openers and needed a victory to put their campaigns back on track.
New Zealand have been India's bogey team in recent global tournaments having prevailed over Virat Kohli's men in the semi-finals of the 2019 ODI World Cup and the final of the World Test Championship this year.
India tweaked their top-order for the match for the same opponents, harnessing Ishan Kishan with KL Rahul in a left-right combination and dropping Rohit Sharma to number three.
Yet they slumped to 48-4 in the 11th over after another top-order meltdown at the Dubai International Stadium.
Trent Boult (3-20) dismissed Kishan for four, and nearly bounced out Rohit with the next ball too but Adam Milne floored the catch at long leg.
Rohit could not capitalise on the reprieve as he and Rahul holed out in the deep.
Leg-spinner Ish Sodhi (2-17) dealt a body blow in the 11th over when he induced a skier from Kohli which Boult pouched at long-on to send groans across the stands populated mostly by India fans.
Hardik Pandya made 23 and Ravindra Jadeja contributed 26 not out as India laboured past the 100-mark after what looked like a catching practice in the deep for the New Zealand fielders.
The New Zealand attack operated with exemplary discipline, conceding only two sixes as they subdued one of the most star-studded batting line-ups of the tournament.
New Zealand did better in their powerplay and reached halfway their target in seven overs losing only the wicket of Martin Guptill (20).
Mitchell missed his fifty by a whisker but Williamson stayed put to hit the winning run.
Pakistan lead the group with three wins in as many matches, followed by Afghanistan who have played an equal number of matches and won two.
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