The Mustard-Onions combination at least ensured a tasty finish for Durham fans before they lost to Derbyshire at Chester-le-Street by two runs.
Replying to t172 for four, Phil Mustard and Graham Onions came together with Durham looking out of contention at 112 for seven in the 15th over.
They got the target down to nine off the last over, bowled by Marcus North, but wicketkeeper Mustard faced only the second ball, driving a single to long-off to remain unbeaten on 67.
When Onions lofted the fourth ball over extra cover for four only three were needed, but he drove the fifth straight back to North then sliced a catch to deep gully to fall for 31 and Durham finished on 170 for eight.
Mustard went in at 50 for five in the ninth over. He smashed three sixes in his 25-ball half-century and added a fourth when he pulled Graham Wagg over mid-wicket.
His place at No 3 was taken by Ben Harmison while David Barrick, a 22-year-old leg-spinner also made his Twenty20 debut. Harmison had to go in at nought for one in the first over and made five in five overs.
After reaching 60 from seven overs, Derbyshire accumulated 59 off the next nine then 53 off the last four, with Hassan Adnan's unbeaten 54 coming off 29 balls. He hit the only six of the innings off Barrick, as the 18th over yielded 20 runs and Adnan's unbroken stand of 76 with Chris Taylor proved decisive.