Bangladesh batsmen Shakib Al Hasan (l) and Mohammad Mahmudullah celebrate after Hasan had reached his century during their partnership during the ICC Champions Trophy match between New Zealand and Bangladesh at SWALEC Stadium on June 9, 2017 in Cardiff, Wales.
In reply, Bangladesh had a nervous start and its four top batsmen fell for just 33 runs.
Despite the win, Bangladesh-who now have three points from three matches-are still uncertain of a semifinal berth and they will have to wait the outcome of the Group A match between England and Australia in Birmingham on Saturday.
Having not given a good account of their abilities in the two games - they lost to hosts England by eight wickets in the tournament opener - Bangladesh's only shot at redemption was the last group match against New Zealand.
Besides Mosaddek's three wickets, Taskin Ahmed, who was playing his first Champions Trophy clash, bagged two wickets while Rubel Hossain and Mustafizur Rahman chipped in with a wicket each.
It was at this very venue nearly exactly 12 years ago - on June 18, 2005 - that a Mohammad Ashraful century had given Bangladesh a shock win over Australia.
Neil Broom, who smashed three boundaries in his 40-ball 36, was another notable contributor for New Zealand.
Shakib was deservingly awarded the man-of-the-match award for his superb innings. Shakib made 114 before getting out while Mahmudullah remained unbeaten on 102. New Zealand played down any frustration after being denied a likely victory over Australia.
2 higher partnerships than Shakib and Mahmudullah's 224 against New Zealand in ODIs.
From Bangladesh, Mosaddek Hossain was the pick of the bowlers with 3 wickets.
When asked if he was luckier than his coach to have the honour of leading a team that loves to break one barrier after another, he said: "I think we are both lucky". Yet a patient 87 from 103 balls was abruptly ended when Williamson was needlessly run out after a breakdown in communication. Williamson went past Nathan Astle (344) and Daniel Vettori (299) today.
"He is doing well there, finishing the game for the team recently, in (the recent tri-series in) Ireland and against New Zealand on Friday, " he added. They managed just 64 runs in the last 11 overs as Mossadek took three wickets, eventually posting a modest total of 265-8.
For the Bangladeshis in the crowd, and indeed across the globe, the win was far more important than a win against a higher-ranked side in an ICC tournament.
That leaves only three other batsmen of the required calibre: Martin Guptil, who has failed to convert his starts into match-shaping scores; Taylor, who has been consistent at a prosaic strike rate; and Williamson, on whom New Zealand are inordinately dependent.
Trent Boult (1-48) had lamented a struggle to find swing at the tournament in the build-up but his bowling partner Southee caused Bangladesh significant problems, accounting for the in-form Tamim Iqbal for a second-ball duck.
Meanwhile, New Zealand will be left to rue another failure to build on impressive displays from Williamson (57) and Ross Taylor (63).
"In saying that we would hjave liked a but more, closer to 300 would have been much better and that was achievable but it wasn't to be". "We were in a position of strength with the bat, and if things were to go our way we know we have a lot of power in that middle, lower order".
Bangladesh skipper Mashrafe Mortaza feels their ICC Champions Trophy experience will help the younger players in the squad prepare better for the 2019 World Cup to be held in England. Twelve years later, in a toe-to-toe battle with New Zealand at the same ground, Bangladesh staged a comeback for the ages, winning by five wickets after being blown away by their pace attack.
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