Defeat by 25 runs to Nottinghamshire was their fourth reverse in seven matches and has left them with a mountain to climb to reach the last eight.
Not for the first time this season, Yorkshire’s batting let them down after they generally performed well with the ball and in the field.
After winning the toss on an overcast day, they restricted Nottinghamshire to 155-8 only to manage 130-6 in reply – and that despite being well-placed at the halfway stage of their innings.
Victory saw Nottinghamshire leapfrog Lancashire at the top of the North Division, Durham climbing third after beating Derbyshire. With only the top two from each of the three groups going through, along with the two best third-placed sides, Yorkshire must overturn a 2.75-point gap to Durham to stand any chance of making progress.
Two points are the prize for a victory in Twenty20, and Yorkshire conclude their group campaign with three tough road trips.
They travel to Lancashire on Wednesday, Nottinghamshire on Friday and finish off with a fixture at Leicestershire on Sunday.
With captain Andrew Gale resting yesterday as a precaution, having experienced slight discomfort in the left hand he broke prior to his return to action against Derbyshire last week, Yorkshire were led by Azeem Rafiq.
They made a good start when former Yorkshire batsman Michael Lumb was caught behind off the final ball of the opening over bowled by former Nottinghamshire pace bowler Ryan Sidebottom, Nottinghamshire slipping to 33-2 in the fifth when Alex Hales lost his off stump when making room to try to whack Liam Plunkett through the offside.
The visitors derived impetus with 17 runs from the sixth over, bowled by Jack Brooks, but lost their third wicket in over number seven when James Taylor pulled a half-tracker from Plunkett to mid-wicket.
Having reached 78-1 at the halfway stage, Nottinghamshire were pegged back by some good bowling by Rich Pyrah in particular, who conceded only 14 from four overs and picked up the wicket of Steven Mullaney, well caught by a diving Dan Hodgson at deep extra cover. Moments earlier, Samit Patel had been caught at long-on off Rafiq after scoring 46 from 37 balls.
When Chris Read was also caught at deep extra cover by Hodgson off Rafiq, Nottinghamshire were 110-6 in the 16th over and some way short of a competitive score. But Ian Butler, the New Zealand pace bowler, thrashed a couple of timely sixes before being run out in the closing stages along with team-mate Graeme White.
David Hussey played a captain’s knock of 52 not out, from 42 balls with four fours and a six, Nottinghamshire scoring 50 runs from the last five overs.
Yorkshire’s reply began poorly when Adam Lyth hit the final ball of Harry Gurney’s first over straight to cover, Joe Sayers and Hodgson then putting them back on track with a stand of 67 in 10 overs.
A couple of mis-fields by Ajmal Shahzad, the former Yorkshire pace bowler, tickled the 7,100 crowd, but their mood quickly changed when Sayers holed out to long-on and Hodgson was stumped to leave Yorkshire 90-3 in the 14th.
Andy Hodd fell lbw and Gary Ballance and Liam Plunkett were caught at deep mid-wicket, Patel and White claiming two wickets apiece as Nottinghamshire closed out the game.