Somerset made a bright start after winning the toss and choosing to bowl, reducing Northants to 26-3 inside five overs with Riley Meredith trapping Ricardo Vasconcelos in front for 15 before Craig Overton removed both Nathan McSweeney (1) and Willey (6) cheaply.
Chris Lynn, the second top run-scorer in this season’s T20 Blast so far, eventually got into his stride for the Steelbacks, hitting three fours and three sixes in a fourth-wicket stand of 65 runs with Saif Zaib to stage a recovery of sorts.
But after Lynn was bowled by Ball’s slower off-cutter for 39 from 26 balls to make it 91-4, the Steelbacks innings failed to gain any real momentum with Somerset taking wickets at regular intervals.
Ball also took the key wickets of Zaib (26) and Lewis McManus (21), while Meredith picked up 3-21 and Overton 2-33.
Northants knew they needed something special and started their defence of 143 perfectly with batter Tom Banton, who was released by England to play on Finals Day, caught behind for a duck off Willey in the first ball of Somerset’s reply.
Thomas Rew, caught by McSweeney off Ben Sanderson, and James Rew, bowled by Willey, quickly followed Banton back to the hut to leave Somerset in real trouble at 12-3 inside three overs.
Will Smeed appeared to have put Somerset back in the driving seat when he smashed Luke Procter for four fours and two sixes in the sixth over to add 28 to the total – more runs than Somerset had managed in their first five overs combined.
But Smeed was dismissed by James Sales for 49, with wicketkeeper McManus taking an excellent one-handed diving catch at full stretch to his right, to stem the flow of runs before Lewis Gregory (13) and Tom Kohler-Cadmore (29) were removed in quick succession to leave Somerset 104-6.
Willey then pulled off the moment of the match in the 18th over with an astonishing piece of fielding to help dismiss Somerset’s quarter-final hero Overton for five, diving over the boundary rope to claw the ball back in mid-air for Zaib to take a catch off Sanderson.
It was a fitting way to cap off Northamptonshire’s resilient performance in the field on the way to victory, which was sealed when Daniel Sams departed for 13 in the 19th over and Meredith was bowled with the last ball of the match by Sanderson, who finished with 3-28.