Worcestershire 255 for 7 (Mitchell 102*, Mennie 3-56) beat Lancashire 254 for 9 (Livingstone 67) by three wickets
Moeen Ali returned to county cricket with Worcestershire but was upstaged by centurion Daryl Mitchell as his side won a last ball thriller against Lancashire by three wickets.
Mitchell rescued Worcestershire from 93 for 6 in pursuit of a 255 target in 48 overs as he reached his 100 off 117 balls with six fours.
He received sterling support from Ed Barnard in a stand of 58 and then skipper Joe Leach in an unbroken partnership of 104 in 17 overs.
Two runs were required from the final delivery from Joe Mennie but he delivered a wide and the two players scrambled the extra run to take Worcestershire over the line.
It was their third win in four matches and dealt a big blow to Lancashire's hopes of reaching the knockout stages.
England allrounder Moeen has spent the last six weeks playing in the IPL and lost his Test place for the opening game of the current series against Pakistan at Lord's which ended earlier today.
The 30-year-old had a modest return of 1 for 49 from nine overs as Lancashire totalled 254 for 9 from 48 overs. He then struck a quickfire 17 but was one of several Worcestershire batsmen out to loose shots.
Australian paceman Mennie's three wicket burst - including Moeen - put Lancashire in the driving seat. But then came the home side's rescue act via Mitchell and Co.
Lancashire were put into bat and Josh Tongue picked up a wicket with his first legitimate delivery of the game after starting with a wide as Keaton Jennings nicked through to first slip.
Alex Davies went for an aggressive swing over midwicket and was bowled by Charlie Morris but then Karl Brown and Liam Livingstone laid the foundations for the Lancashire innings with a stand of 76 in 13 overs.
Livingstone lofted Barnard over midwicket for a towering six before he made a double breakthrough. Brown went for an expansive drive and was bowled and Dane Vilas perished in the same fashion two balls later.
Livingstone went to his half century off just 48 balls with one six and three fours but after making 67 he fell victim to a brilliant full length catch by keeper Ben Cox off Tongue.
After a brief delay for safety reasons because of nearby lightning, the visitors innings lost momentum as Jordan Clark was lbw to Joe Leach and Brett D'Oliveira had Josh Bohannon and Mennie also leg before.
Moeen got in on the wicket-taking as Steven Croft was caught behind but Graham Onions (30 not out) and Matthew Parkinson added an unbroken 38 for the final wicket.
Onions then quickly made an impact with the ball Worcestershire quickly lost opener Joe Clarke leg before in his first over. But it was his new ball partner Mennie who made decisive inroads into the innings.
His South Australian team-mate Travis Head - in the last match of his current spell with Worcestershire before joining the ODI squad to face England - was bowled for 17 after aiming a big hit over midwicket.
Moeen pulled a couple of boundaries off the 29-year-old through the leg side but then tried to repeat the same shot on 24 and holed out to midwicket.
Mennie collected a third scalp as Brett D'Oliveira picked out Alex Davies on the deep midwicket boundary. Jennings then got in on the act as Ben Cox departed to a stunning catch by Croft at point and Ross Whiteley went for a pull and edged through to keeper Vilas.
But then Mitchell and Barnard joined forces to launch the recovery. Barnard was eventually lbw to Matt Parkinson but Leach (42 not out) also proved a valuable ally to Mitchell.