Chasing a modest 223, India were given a flying start by openers Rishabh Pant (51 off 26 balls; 9x4, 2x6) and Ishan Kishan (24 off 14 balls; 2x4, 2x6) who together put on 67 from 33 balls before Bangladesh triggered a collapse after the latter was run-out following a mix-up.
From 67 for no loss, India were reduced to 116/4 in 21.2 overs but the duo of Sundar (50) and Amandeep Khare (41) held the innings together in a 69-run partnership to set up the home side's victory with nine balls to spare at Jadavpur University second campus ground.
"It's the most important position but I knew there was no extraordinary thing needed and had to bat normally. I was confident and went with a positive mindset," Sundar said.
Asked on coach Rahul Dravid's influence on the team, he said: "He's giving positive vibes to everyone. It's great to have a legend like him as the coach."
India now have 13 points from three matches to seal their berth in the Sunday's summit clash, while Bangladesh have five points by virtue of a win.
In their last league match tomorrow, Bangladesh will take on Afghanistan who are ye to to open their account.