Simon Harmer and Aron Nijjar shared seven wickets to spin Essex Eagles past Derbyshire Falcons and into the Vitality Blast final at Edgbaston.
Defending 160-5, Essex bowled Derbyshire out for 126 to win by 34 runs with the spinners doing the bulk of the damage.
Harmer (4-19) produced two beauties to bowl Billy Godleman and Leus du Plooy and when Anuj Dal walked past another from the off-spinner, the Falcons crumbled and were eventually bowled out in the 19th over.
Essex will now face Worcestershire in the final after the Rapids won a thrilling first semi-final against Nottinghamshire earlier in the day.
Having elected to bat, Essex got off to a flyer as Cameron Delport and Tom Westley brought up the fifty partnership in the final over of the powerplay.
Delport (55 from 31 balls) was finding the boundary with great regularity, hitting nine fours on his way to a 28-ball half-century, but did not get much further as he holed out to long off, Alex Hughes the bowler to break the 78-run opening stand.
Pace off the ball was working for Derbyshire and they dragged themselves back into the match as Hughes (2-24) got rid of Dan Lawrence (3) before Luis Reece (2-24) removed Ryan ten Doeschate (1) and Westley (39) to leave Essex 107-4 in the 14th over.
Ravi Bopara and Adam Wheater (20no) got the Eagles back on track and although Bopara (27) fell to Ravi Rampaul (1-30) late on, Essex's approach in the second half of the innings suggested they were confident they had a strong total on a dry Edgbaston wicket.
Reece made a positive start for Derbyshire in their chase, thumping three fours and a six only to have his fun cut short midway through the second over by a superb one-handed catch from Wheater behind the stumps off Aaron Beard (1-6).
Harmer brought himself on to bowl the fourth over and struck with his first ball, ripping the ball past Godleman's (9) outside edge to hit off stump. He followed that up with two in two balls in his third over, another stunner turning from leg to hit off and bamboozle Du Plooy (9) before Dal (0) was castled after advancing down the track.
Nijjar had seen his first four balls go for 14 but fought back to bowl Wayne Madsen (17) round his legs and when the left-arm spinner bowled Matt Critchley (5) in the 11th over, Derbyshire were all but beaten at 81-6.
Hughes (23) was stumped off Nijjar (3-26) to make that 86-7, part-time off-spinner Lawrence (1-20) chipped in with the wicket of Fynn Hudson-Prentice (1) and Harmer returned to have Daryn Smit (19) caught, leaving it to Delport (1-11) to finish it off by having Rampaul (6) caught in the deep.
Watch continued live coverage of Vitality Blast Finals Day, with the final between Essex Eagles and Worcestershire Rapids, live on Sky Sports Cricket from 6.15pm.