Durham 288 for 8 (Borthwick 76) beat Middlesex 225 for 9 (Handscomb 75) by two runs (DLS method)
Durham skipper Scott Borthwick top-scored with 76 and then delivered a crucial over to secure his side's second straight Royal London Cup success as they edged a rain-affected clash with Middlesex at Radlett.
Borthwick struck 76 in Durham's total of 288 for 8, supported by half-centuries from Graham Clark and Luke Doneathy, only for Peter Handscomb and Robbie White to put Middlesex on track for victory with a fourth-wicket partnership of 130.
But both fell in quick succession just before rain forced the players from the field at 173 for 5 from 32 overs - which was enough to push the home side behind under Duckworth-Lewis-Stern calculations.
They returned for a further four-over thrash - with Borthwick conceding just four from his six balls and, despite a valiant flurry of boundaries by Thilan Walallawita, Middlesex fell just short of the revised target of 228.
Clark - who had struck a career-best 141 against Kent two days earlier - continued his form when Durham won the toss and batted, hammering three boundaries from Tim Murtagh's opening over.
Despite the early loss of Alex Lees, well held by Sam Robson at midwicket, Clark and Borthwick scored freely in their partnership of 108, aided by some ragged Middlesex outfielding.
Luke Hollman spilled a pull from Clark on the boundary and the opener progressed to 65 before he went after Robson's long-hop and picked out the diving Handscomb in the deep.
Hollman made amends for his earlier drop by removing Cameron Bancroft for a second-ball duck, but Borthwick found a willing ally in David Bedingham, who pummelled 41 from 31 deliveries to dominate their brisk stand of 66.
At 193 for 3, Durham looked on course to post a daunting total, only for Borthwick to squander his opportunity of a maiden List A hundred by holing out off Murtagh and Bedingham departed in the next over, caught behind off James Harris.
The visitors' innings threatened to stall entirely, but a fearless knock by 20-year-old Doneathy - who clubbed five fours and a sweetly-struck straight six off Harris in his maiden 50 from 48 balls - gave them renewed impetus.
Middlesex were soon floundering at 37 for 3 in reply, with Max Holden falling to Jack Campbell and Varun Chopra lbw shuffling across against Chris Rushworth, who then bowled Robson with one that nipped back to hit off stump.
Handscomb and White undertook the reconstruction job, with the skipper employing the sweep shot to good effect and using his feet against spin in a knock of 75 from 71 balls.
White opted for placement rather than power and picked the gaps consistently, reaching his first 50-over half-century with a leg-side boundary off Borthwick - but Campbell made the breakthrough, returning for a second spell to have him caught behind for 55.
When Handscomb fell in the next over, Durham shot ahead under the DLS calculation - and that was how it remained when a downpour descended on the ground soon afterwards.