An outstanding innings from Phil Mustard set Durham up for a crushing victory over Surrey at Chester-le-Street in the CB40.
Surrey had been overtaken at the top of Group B by Hampshire the day before, and their chances of returning to the top were shattered as Mustard hammered 143 off just 91 deliveries.
Durham's total of 298-9 looked like being even more daunting with Mustard in blazing form but his departure led to a mini collapse as just 27 came from the last six overs.
Mark Stoneman set a platform for Mustard with a run-a-ball 50 and Ben Stokes contributed 45 in a second wicket partnership of 119 in just 14 overs.
Zander de Bruyn's medium pace picked up 5-46 to briefly give Surrey some hope, but Rory Hamilton-Brown and Steven Davies were gone after two overs of their reply and the innings never recovered from 10-2.
Mark Wood (3-32) worked his way through the middle order before Gareth Breeze wrapped things up with three wickets in four overs at the end of the innings, only Jason Roy making any real contribution with 43 off 24 balls.