Yorkshire 183 for 4 (Lees 67, Williamson 41) beat Derbyshire 124 (Cross 37, Rafiq 3-26) by 59 runs
Yorkshire planted one foot firmly in the quarter-finals when they crushed Derbyshire by 59 runs with 15 balls to spare in a one-sided game at Chesterfield.
Alex Lees continued his remarkable run against Derbyshire with a Twenty20 career-best unbeaten 67 - he has now scored 403 in 3 innings in all cricket against them - as Yorkshire raced to 183 for 4 from their 20 overs.
Kane Williamson made 41 off 26 balls and skipper Andrew Gale smashed a rapid 34 as 68 came off the last five overs and, although Gareth Cross hammered 37 from 19 balls, Derbyshire collapsed to 124 with five Yorkshire bowlers striking in their first over as nine wickets fell to spin.
Williamson put down a marker by driving and cutting Wes Durston for three fours in the opening over and, although Aaron Finch never got going, Lees took full advantage of an early life to hammer the attack on the ground where he made an unbeaten 275 in the LV= County Championship last season.
Lees was on one when he drove former Yorkshire spinner David Wainwright to long-off where Alex Hughes misjudged the chance and could only push the ball over his head for six. The next ball vanished over the pavilion and when Lees drove Chesney Hughes back over his head, it was his third six in six balls but Derbyshire stemmed the flow of runs with Marcus North bowling Williamson in the ninth over.
North then had Johnny Bairstow stumped swinging to leg and the tempo dropped until Gale smashed the Australian through the covers for the first boundary in 5.2 overs. That opened the floodgates for a second time as Lees and Gale plundered 65 from 38 balls with Lees reaching 50 by hitting Durston back over the bowler's head for his third four.
Gayle holed out to deep square leg after hitting four fours and a six from 20 balls but Tim Bresnan made sure Derbyshire would face a daunting chase by driving Mark Turner for six as 18 came from the last over.
Derbyshire lost Durston who was bowled sweeping at the second ball of their innings but Cross pulled Bresnan for six and hoisted Williamson onto the top of a refreshment wagon before driving him into the crowd over long-off. The former Lancashire wicketkeeper was threatening to spoil Yorkshire's afternoon until he pulled Richard Pyrah high to deep mid-wicket where Williamson took the catch just inside the rope.
With Cross gone, Yorkshire got the game back under control and with the rate climbing, Wayne Madsen chipped Azeem Rafiq to long on and Billy Godleman was stumped charging the offspinner as Derbyshire limped to 67 for four after 10 overs.
North was caught and bowled by Adil Rashid for 20 two runs later and Alex Hughes became Rafiq's third victim when Bresnan took a diving catch at deep midwwicket.
Although Tom Knight hit three sixes to give the Derbyshire fans something to cheer, the game had long gone and Derbyshire's 10th defeat of the season was inevitable long before Turner chipped Adam Lyth to mid-on.
"I thought the way Alex Lees batted was nothing short of sensational," Yorkshire's first team coach Jason Gillespie said. "He read the situation very well and made sure he was there at the end. At the end of the day you have to get your skills right and the way we bowled and fielded was fantastic.''
Derbyshire skipper Wayne Madsen said: "They probably got 25 too many and although we got a good start, when the spinners came on it was pretty difficult to score. The innings by Alex Lees changed the game, he struck the ball extremely cleanly and while it was not good to be on the other end of it, it was good to watch."