Patel’s brilliant maiden one-day century lifted the visitors from 102 for eight to 188 all out, and he then sparked Durham’s collapse from 127 for one to 177 for nine.
For the second successive year against Nottinghamshire in this competition it was Gareth Breese who secured Durham’s one-wicket win.
Last year the Jamaican all-rounder hit 68 not out and this time he was unbeaten on 34 after driving the last ball of the 49th over for the winning six off Darren Pattinson.
With 12 needed off two overs, only two runs had come off the first four balls, but Breese drove a full toss wide of mid-off for four before his winning hit.
Patel, 23, took three wickets with his left-arm spin, with the first one a return catch to get rid of former team-mate Will Smith following a second-wicket stand 107 with Michael Di Venuto.
In the next over Di Venuto fell for 70 when he drove Pattinson to extra cover then skipper Dale Benkenstein looked aghast when he was adjudged to have edged Patel to Chris Read.
With Neil McKenzie hitting Pattinson straight to midwicket, Durham lost five wickets for 12 runs after needing 62 off 20 overs with nine wickets standing.
Breese joined Ben Harmison at that point and they put on 24 before Harmison holed out to deep midwicket.
Mark Ealham then weighed in with two wickets, but Mark Davies, who had been off the field through illness, kept Breese company while the last 12 runs were scored.
Nottinghamshire chose to bat in sunny conditions, but their innings was a one-man show as Patel hit 17 fours and two sixes in scoring 114 off 113 balls.
His innings began quietly with his side on 34 for four before exploding in thrilling fashion as his second 50 came off 30 balls. He put on 86 for the last two wickets with Pattinson and Charlie Shreck, who contributed nine between them.
With Davies off the field and unable to bowl his last two overs, Durham turned to Ben Harmison to bowl the 48th over and after his first ball disappeared for four the next was a straight yorker, which Patel tried to paddle to fine leg and missed.
That left 16 balls of the Nottinghamshire innings unused, which proved crucial.
Durham now wait unitl the draw during lunch of Englands test match to see who they face in their second semi final in two years.