The Dynamos had expected to play Yorkshire in the quarter-finals a fortnight ago, but the White Rose county were eliminated after fielding an ineligible player, allowing first Nottinghamshire and then Glamorgan to be reinstated.
The Welsh side initially seemed intent on making the most of their reprieve and booking their place in Saturday’s finale to the competition.
But Durham overcame a sluggish start at the Riverside to post 163 for eight, Will Smith hitting 51, which was enough for them to win by 44 runs.
There was slight controversy at the toss as Glamorgan skipper David Hemp appeared to see the coin land before calling.
He decided to put Durham in and for the majority of the hosts’ innings it looked a smart move.
Ultimately, however, the Dragons’ decision not to play former Australia paceman Jason Gillespie may have cost them as they conceded 55 from the final four overs.
Gillespie was left out due to concerns his involvement with the Indian Cricket League could deny the Dragons the chance to play in the £2.5million Champions League, should they reach the final.
It was not until Smith and Dale Benkenstein put on 53 for the fifth wicket that Durham began to build any momentum.
They lost Phil Mustard in the second over, bowled trying to hit James Harris across the line, and axed England all-rounder Paul Collingwood soon followed as he was trapped leg before by David Harrison.
Former West Indies captain Shivnarine Chanderpaul then scooped Harris to mid-on and when Michael di Venuto chipped back to Robert Croft for a well-taken caught and bowled, Durham were toiling at 49 for four.
Smith began the revival when he clubbed Dean Cosker over cow corner for six and hit another five boundaries on his way to a 37-ball half-century.
He was dropped on 37 by Cosker but the fielder had his revenge when Smith was sent back chancing a risky single and run out by a direct hit.
By that time, captain Benkenstein had also perished having holed out to Tom Maynard at long-on off Alex Wharf.
The Dynamos were in danger of falling well short of par until Gareth Breese, on his way to a nine-ball 20, smashed two fours and a six as the 18th over off Wharf went for 19.
He found Maynard on the boundary in the next over but Shaun Pollock (20) and Liam Plunkett (12 not out) combined to take 22 off the last from Harris.
Glamorgan were in trouble from the first ball of their reply as Plunkett removed Richard Grant’s off stump and Hemp edged behind to Mustard later in the same over.
Plunkett claimed his third wicket as Smith took a fine low catch to remove Mike Powell, and Maynard was unfortunately run out after Jamie Dalrymple’s drive struck Steve Harmison and hit the stumps.
Dalrymple (32) offered brief hope but chipped Collingwood to Benkenstein, and Harmison emphatically ended Mark Wallace’s attempts to save his side, bowling him for 26.
Glamorgan then collapsed from 88 for six to 118 all out, Collingwood and Pollock finishing with two wickets apiece and Plunkett three for 16.