Leicestershire 149 for 5 (Kimber 40, Cox 38*) beat Lancashire 145 for 7 (Jennings 39, Green 36, Rehan 3-16)
Rehan Ahmed put Lancashire Lightning in a spin with a superb three for 16 from four overs and later a crucial 27 as Leicestershire Foxes breached fortress Emirates Old Trafford to claim an impressive Vitality Blast victory.
Legspinning allrounder Ahmed is due to link up with England for their T20I series against the West Indies later this week, and he claimed season's best figures across all formats, limiting the Lightning to 145 for seven on a used pitch.
He bamboozled a home batting line-up which saw opener Keaton Jennings top-score with 39 off 19 balls before Chris Green added a crucial 36 not out off 29 late on.
The 20-year-old then came in with Leicestershire's chase in limbo at 47 for three in the sixth over and contributed again to a five-wicket win with nine balls remaining, hitting two fours and a six in 23 balls.
The Foxes have now won two and lost one at the start of the North Group, while Lancashire's three-game winning start came to an end with a first defeat.
In T20 history, Lancashire have won 89 of 124 matches at Emirates Old Trafford, losing only 31. But Leicestershire are the most successful visiting county here, winning on six occasions since 2003.
Six of the seven home wickets fell to spin, giving the Red Rose, who later employed four spinners, a taste of their own medicine.
Ahmed bowled Michael Jones, Australian overseas Ashton Turner and Tom Hartley as the score fell to 114 for seven after 17 overs.
Left-armer Liam Trevaskis also impressed with two for 17 and captain Louis Kimber struck once with his part-time offies.
Kimber made the initial breakthrough when Luke Wells pulled to deep midwicket - 15 for one in the second over.
Captain Jennings then advanced Lancashire to 45 for one in the fifth over. He drilled Kimber for an arrow straight six added to six boundaries before slicing Matt Salisbury's seam to short third.
From there, wickets fell regularly. At its worst, Lancashire were 95 for six in the 14th over.
Aussie all-rounder Green then accumulated his 36 with only one six and a comical all-run four off the last ball of the innings which came as the ball pin-balled around Leicestershire's fielders.
Green then played a big part at the start of the Foxes' chase, striking twice with his off-spin as the score slipped to 47 for three in the sixth over.
He trapped Rishi Patel lbw and bowled Shan Masood. Sandwiched in between, Sir James Anderson had a pulling Sol Budinger caught at deep backward square-leg.
But Ahmed was to the fore once more.
Having come in at number five, he saw captain Kimber fall for a quick-fire 40 off 22 balls to Hartley's left-arm spin - 63 for four in the eighth over - before lofting Charlie Barnard's spin over cover for six.
And while Barnard later got revenge by bowling him, the damage had been done in a 59-run partnership with busy wicketkeeper-batter Ben Cox, who was also excellent for a run-a-ball 38 not out.
