Captain Lewis Gregory hit 60 from 24 balls to help Somerset reach T20 Finals Day with a 19-run win over Notts Outlaws.
James Hildreth made 52 off 28 and Tom Abell 46 as the hosts posted 209-5 from 20 overs at Taunton.
Alex Hales’ 45 got Notts off to a strong start, but his dismissal by Jamie Overton slowed their charge.
Overton ended with 5-47 as Notts were bowled out for 190, earning Somerset a semi-final against Sussex Sharks.
Lancashire Lightning will play Worcestershire in the other semi-final on Finals Day, which is being held at Edgbaston on 15 September.
The Somerset-Nottinghamshire game was scheduled for Sunday but heavy rain pushed it back to Monday’s reserve day.
Gregory reached his fifty from only 20 balls, despite being doubt to play because of a groin injury suffered in the County Championship win over Essex.
England batsman Hales was dropped on 33 by Abell off Gregory, who took 2-29, before he drilled a wide Overton delivery to Gregory at cover.
Nottinghamshire were behind the required run-rate from then on, and Jerome Taylor bowled Harry Gurney with the final delivery of the match.
Somerset bowler Jamie Overton told BBC Sport:
“Obviously it was a mixed sort of a day for me, it was just that I was bowling to one of the best batsmen in the world in Alex Hales.
“He was a number one for a reason and can be so destructive at times. I bowled the balls I wanted to but he kept hitting them for four. It was massive for us to get him out when we did.
“Lewis coming in, when he did, and getting 50 off 20 balls shows how deep we bat. He’s had a phenomenal year for us, with runs and wickets in all competitions.
“We haven’t been to Finals Day for a few years, so it will be nice for me to get there and see what the atmosphere is all about. Obviously we beat Sussex at Hove recently so we go there with a positive attitude and hopefully we can put one over them again.”
Notts Outlaws head coach Peter Moores:
“It was a funny game in some ways. I think we did really well but they got away right at the end of their innings and that hurt us in the long run.
“We have no complaints. It’s a tough place to play and Lewis Gregory had a great game; he batted beautifully at the end and Tom Abell also played nicely.
“I think it was chaseable and we got off to a decent start but then we just got a little bit stuck towards the middle and once we’d lost wickets it was always going to be tough chasing a score like 210.”