Middlesex 275 for 6 (Morgan 90, Simpson 58*) beat Leicestershire 274 for 8 (Eckersley 73, Smith 56, Boyce 53) by four wickets
Acting Middlesex captain Eoin Morgan warmed up for England's forthcoming limited-overs series against Australia with a season's best 90 to inspire his side's four-wicket Yorkshire Bank 40 win over Leicestershire at Lord's.
Although neither side could progress from Group C, Morgan caught the eye of England's national selector Geoff Miller - watching from the media centre - by hitting eight fours and four sixes in a match-defining innings as Middlesex reached their target with10 balls to spare.
In pursuit of Leicestershire's impressive 40-over total of 274 for 8, Middlesex struggled early on under the pressure of an asking rate of 6.88 an over and some tight Leicestershire bowling.
Leading scorer Dawid Malan had still to get under way when he followed a delivery from left-arm seamer Rob Taylor to edge to the wicketkeeper. Then, with the total on 22, Paul Stirling aimed an expansive cover drive at Nathan Buck only to drag the ball onto middle stump.
The experienced third-wicket pairing of Joe Denly and Morgan regrouped with a partnership worth 72 inside 11 overs that came to a surprise end when Denly chipped a return catch to diving slow left-armer James Sykes.
Morgan reached a 43-ball 50 - his first in all forms of cricket in a hugely disappointing domestic season - by chipping six into the Tavern Stand against Josh Cobb. He was just 10 short of his century when a mistimed slog sweep against Shiv Thakor flew to Greg Smith at long-on.
Morgan helped add 81 in 10.3 overs for the fourth wicket with Adam Voges during a breezy 63-ball stay, but Voges blotted his copybook by holing out to the same fielder to make it 197 for 5.
Needing 77 from their final 10 overs, Middlesex added 60 in 44 balls through debutant Ollie Wilkin and John Simpson before the former cut to backward point with only 18 required. Simpson saw the hosts home with a brisk unbeaten 58 from 39 balls with a six and eight fours, the last of which won the game.
Batting first after losing the toss on a bright St John's Wood afternoon, Leicestershire lost Cobb for six when he miscued to mid-off against England paceman Steve Finn.
Leicestershire opener Smith moved smoothly to a 61-ball half-century with four fours and two sixes. But only 11 balls later, Smith perished when attempting to clip spinner Ravi Patel over long-on, spooning to long-off to end a second-wicket stand worth 61 in 13.1 overs with Ned Eckersley.
Eckersley was joined by left-hander Matthew Boyce, who took advantage of a short boundary on the Tavern side by twice depositing short deliveries from Patel directly into the Mound Stand.
Eckersley also reached 50 from 53 balls and with eight fours but the former MCC Groundstaff player top-edged his pull shot high to Toby Roland-Jones at mid-on to depart for 73. Boyce, having added 95 in 12.2 overs in tandem with Eckersley, became the third visiting player to pass the 50 milestone, his 50 coming from 51 balls.
Rookie Middlesex seamer Wilkin claimed his maiden YB40 wicket by removing Boyce for 53 then, four balls later, the 21-year-old made it two for 44 when Michael Thornely flailed outside off to be caught behind for 19. The visitors lost three for 11 in six balls in the quest for late runs, which worsened when Ben Raine was caught at deep extra cover for seven.
Yet Middlesex were docked six penalty runs for a slow over rate going into the final over by Harris, which also led to the departure of Thakor to a comfortable catch at long-leg by Finn. The last five legal deliveries of the innings cost 24 runs as Tom Wells threw the bat to plunder two sixes and three fours in a cameo unbeaten 31.