Northamptonshire 193 for 8 (Crook 56*, Kleinveldt 42) beat Worcestershire 191 for 6 (Whiteley 41*) by 2 runs
The eighth-wicket pair thrashed 62 from 3.1 overs before Kleinveldt was caught on the straight boundary from the last ball of the Steelbacks innings after hitting six sixes in a match-turning 42 from 12 deliveries.
Having lifted the Steelbacks to a formidable 193 for 8, the South African led the next phase with the first-ball dismissal of Moeen Ali before the Rapids were held to 191 for 6 as they narrowly failed to extend a three-match winning run.
Moeen's exit capped a night of mishaps for England players. David Willey's return from the one-day internationals ended in discomfort after damaging an ankle when bumping into Jack Shantry while making 31 from 12 balls at the start of Northants' innings.
Willey's injury to his right ankle was a worry for Northants head coach David Ripley. He said: ``We have got our fingers crossed. He has gone off for an X-ray just to check his bone. He's kind of gone over a bit on his foot. He is booked in for a scan on Monday. Maybe it will be a twisted ankle and he'll be back firing for us next week.''
With Willey unable to bowl when the Rapids batted, Moeen bowed out tamely when driving to extra cover in his final county appearance before joining England's pre-Ashes training camp in Spain this weekend.
Perhaps it was a sign of the pressure that came from losing control in the late stages of the Northants innings. If Kleinveldt was the supercharged force, Crook was not far behind with an undefeated 56 which came from 35 balls as the Steelbacks kicked on from 131 for 7 in the 17th over.
The transformation began when Kleinveldt launched a sequence of four, six, six from Saeed Ajmal's last three balls. Shantry's final over then cost 24 runs and Kleinveldt spoiled Ed Barnard's figures with three consecutive sixes from the closing over.
The Steelbacks also made a flying start. Richard Levi made 19 from 11 balls until being caught at deep mid-wicket and Willey hit two sixes before limping out of his accidental collision with Shantry. After treatment, he cleared the rope twice more before top-edging Ajmal to short fine leg.
The middle of the innings was less convincing with only Alex Wakely making significant progress against Worcestershire's three spinners. After brilliantly running out Josh Cobb, Brett D'Oliveira bowled his four overs for 26 runs and one wicket.
Worcestershire made a bold attempt to keep up with the rate in a partnership of 76 in just short of eight overs by Daryl Mitchell and Tom Kohler-Cadmor, but wickets began to fall, most damagingly when Colin Munro was caught at long-off in a brilliant over from Muhammad Azharullah.
Still the Rapids kept going with an unbeaten 41 from 39 balls by Ross Whiteley and support from Ben Cox in a late stand of 54.
However, man of the match Kleinveldt had the last word with two crucial overs in which he dismissed Cox and conceded only one boundary. With 14 wanted from the last over, Whiteley drove Azharullah's first five balls for twos but could only manage a leg-bye from the last.