Sri Lanka chased down the 205-run target with an over to spare, thereby maintaining there unbeaten streak in the tournament, after Bangladesh opted to bat in the inconsequential match at the Shere Bangla National Stadium.
Having already made the final alongside Pakistan, Sri Lanka were in big trouble at eight for three in the third over, before being reduced to 75 for five at the midway stage by a spirited Bangladesh attack.
But Mathews (74) did not panic as he struck seven boundaries and one six en route to his 19th ODI half century, finishing the chase by pulling Arafat Sunny for a boundary.
The skipper found a perfect ally in Chaturanga de Silva (44 from 52 balls; 5x4) and the duo stitched 82 brisk runs for the sixth wicket.
With four wins from as many matches, the Lankans finished atop the table with 17 points, while Pakistan finished with 13 points. India (9) and Afghanistan (4) ended at third and fourth places, while hosts Bangladesh finished with the wooden spoon with four defeats.
The summit clash will be held here on Saturday. Earlier, Bangladesh frittered away a solid start by their openers to be restricted to 204/9 after Mushfiqur Rahim opted to bat on an uneven looking slow surface.
Rookie Priyanjan, with his off-spin, grabbed 2/11 from three overs after Mendis gave the breakthrough with two wickets from one over. Bangladesh's opening duo had put on 74 runs.
Opener Anamul Haque, who scored a century in their record 326 against Pakistan, struck one six and two fours en route to an 86-ball 49.
In the next over, Priyanjan took the prized scalp of Shakib-al-Hasan (20), while the pace duo of Suranga Lakmal (2/32) and Thisara Perera (2/29) tightened the noose in the death overs.
In their defence, Bangladesh came out all guns blazing as Al-Amin Hossain dismissed Kusal Perera for a duck in the second ball of Lankan innings, before grabbing the prized scalp of Kumar Sangakkara (2) in his next over.

Teams (From):  
Shamsur Rahman, Anamul Haque (WC), Mominul Haque, Mushfiqur Rahim (C), Shakib Al Hasan, Nasir Hossain, Mahmudullah, Ziaur Rahman, Arafat Sunny, Rubel Hossain, Al-Amin Hossain
Sri Lanka: MDKJ Perera, HDRL Thirimanne, KC Sangakkara (WC), DPMD Jayawardene, SMA Priyanjan, AD Mathews (C), PC de Silva, NLTC Perera, SMSM Senanayake, BAW Mendis, RAS Lakmal.


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