Durham 215 for 6 (Mustard 91) beat Yorkshire 139 for 8 (Ballance 68) by 76 runs
Phil Mustard hammered Durham's highest individual Twenty20 score of 91 in a 76-run home victory against Yorkshire at Chester-le-Street.
Mustard struck 11 fours and three sixes in scoring his runs off 52 balls as Durham amassed 215 for 6, their joint second highest T20 total.
Without their England men and injured skipper Andrew Gale, Yorkshire quickly slipped to 22 for 3 in reply and finished on 139 for 8. Gary Ballance made 68 and the second-highest scorer was acting captain Azeem Rafiq with 15 not out.
Durham completed the double over Yorkshire but have lost their other three games and both teams are struggling to progress in the competition.
Durham's previous best was 83 not out, scored by Herschelle Gibbs against Derbyshire last year, and their first century looked certain until Mustard tried to run Rich Pyrah to third man and chopped the ball into his stumps in the 14th over.
Rafiq put Durham in and after they took only 18 off the first three overs Mustard hammered 27 off the fourth, with the help of a no-ball from Jack Brooks. Sixes over long-on and midwicket were followed by fours to fine leg, long-on and backward point.
The score had reached 62 in the sixth over when Mark Stoneman clipped Pyrah to short fine leg to depart for 12.
Liam Plunkett proved hard to get away in his first two overs until Mustard hooked his 12th ball behind square for his eighth four to reach 50 off 26 balls. Scott Borthwick helped to maintain the tempo, hitting Rafiq over midwicket for six and the stand was worth 83 in eight overs when Mustard was out.
Borthwick holed out for 35, off 18 balls, then Ben Stokes hit four successive balls from Plunkett to the boundary. He fell for 23, but Gordon Muchall kept the runs flowing, his unbeaten 27 coming off 16 balls after 17 came off the final over from Plunkett.
Yorkshire's top three totalled five between them as Chris Rushworth persuaded Phil Jacques to clip to midwicket and Dan Hodgson to drive to mid-off, while Joe Sayers was bowled by Stokes.
Ballance reached 50 off 31 balls but finally skied a catch to extra cover off Gareth Breese and Yorkshire could do little more than play out the overs.