New ball pair Tim Southee and Trent Boult, for the second consecutive game, were strong in the early overs to help restrict Mumbai to 132 for nine. Kane Williamson and Anton Devcich put on 83 in 9.4 overs to lay a platform for victory. Williamson's 53 off 36 balls was his second half century in three games.
"It was a hell of a performance by the boys," former international Flynn said.
He added the ND team were enjoying the Raipur ground, where around 45,000 people turned up on one day during the qualifying event.
"The conditions seem to suit us," he said. "There's a little bit there for the seamers and a little bit for the spinners, but the way our guys have played the spin throughout the qualifying games has been a pretty important part to getting where we are."
Not having to leave Raipur for another two games won't hurt.
Continuity, and strong performances from senior players have made ND a decent prospect to make the last four.
ND's final group game, against Barbados Tridents, will be the final pool game, leaving them knowing exactly what is needed to make the semifinals.
The top two from each group make the semifinals in Hyderabad early next month, with the final in Bangalore on October 5 (NZT).
What Lies Ahead
• Northern Districts start their group B games against South Africa's Cape Cobras in Raipur early on Saturday (NZT)
• They face the Hobart Hurricanes, also in Raipur, early next Wednesday, followed by the Kings XI Punjab in Mohali on September 27 and finish with the Barbados Tridents in Bangalore on October 1.