After restricting Bangladesh to a below-par 245 for nine, Mitchell and Williamson led the chase, sharing a solid 108-run stand. Opener Devon Conway too hit an impressive 45 as the Kiwis chased down the target with 43 balls to spare.
However, Williamson, who missed out on the first two games because of his knee injury, suffered another one his left hand and had to retire on 78 during the chase.
Playing in his first match of the ongoing showpiece after finally recovering fully from the knee injury he had sustained during the IPL in April and the ensuing surgery, Williamson showed his class with an old-fashioned calculative knock. At the other end, Daryl Mitchell (67-ball 89) looked in a hurry as he smashed fours and sixes on his way to a fiery 67-ball 89.
As It Happened
Having found two good-looking boundaries at the start of his innings, Williamson decorated his knock with eight fours and a six, while the Mitchell hammered six fours and four sixes during his breezy, unbeaten stay in the middle.
Restricted to 245 for nine after being asked to bat first, Bangladesh bowlers had no answer to the Kiwi response but they did a few things wrong, including their skipper and biggest weapon with the ball Shakib al Hasan completing his quota of overs instead of keeping them for the end overs.
The New Zealand dressing room, however, was not happy to see an exhausted Williamson walking back retired hurt after attempting a flashy drive. This was after he had taken a blow to his left thumb.
New Zealand, by then, had done enough to ensure victory.
Earlier, Mushfiqur Rahim top-scored with 66 as Bangladesh battled hard in the face of New Zealand’s disciplined bowling effort to post 245 for nine.
Mushfiqur’s 75-ball knock, alongside valuable contributions from Shakib (40 off 51 balls) and Mahmudullah (41 off 49), lifted Bangladesh after they were reeling at 56 for four in the 13th over.
The Tigers began on a disastrous note, losing Litton Das (0) off the very first delivery of the game to pacer Trent Boult, as his attempted flick was collected by Matt Henry in fine leg.
Tanzid Hasan (16 ) and Mehidy Hasan Miraz (30) tried to put up some resistance before the former fell prey to pacer Lockie Ferguson, offering a catch to Devon Conway in square leg.
Notably, Tanzid was dropped when he was yet to open his account by Tom Latham in the opening over itself. It was a tough chance, nonetheless.
When the score read 56, Bangladesh lost Miraz to Ferguson, followed by Najmul Hossain Shanto (7) to spinner Glenn Phillips.
Losing two wickets in a space of three balls was a body blow for Bangladesh, only for Shakib and Mushfiqur to take control for some time with a 96-run partnership for the fifth wicket.
However, Shakib appeared to be struggling in the hot and humid conditions here, taking a medical break before eventually being dismissed by Ferguson in the 30th over.
The pitch appeared to be assisting the pacers, and Ferguson looked to capitalise with his ‘bowl short and quick’ strategy.
Also, the strip utilised was different from the one that helped the spinners during the India-Australia contest on Sunday.
Meanwhile, Mushfiqur brought up his 48th ODI half-century and was cleverly attacking the loose deliveries, while spinners Mitchell Santner and Rachin Ravindra bowled tight lines.
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In the 36th over, pacer Matt Henry gave the Kiwis relief as he bowled Mushfiqur with a beautiful slower off-cutter that stayed low and uprooted the off-stump, leaving Bangladesh at 175 for six.
After losing the seventh wicket in Towhid Hridoy for 13, Mahmudullah and Taskin Ahmed (17) added 34 runs for the eighth wicket before the latter gave a catch to Daryl Mitchell off Santner.
While Mustafizur Rahman was the ninth man out in the 48th over, the final pair managed to bat out the remaining deliveries.
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Ferguson took 3 for 49, while Boult grabbed 2 for 45, including his 200th ODI wicket. Meanwhile, Santner was the most economical of them all, finishing with figures of 1/31 in 10 overs.
(With inputs from PTI)