Lightning 320 for 6 (K Bryce 162, S Bryce 90) beat Sparks 296 for 6 (Kelly 62, Jones 60) by 24 runs
A brilliant career-best 162 from skipper Kathryn Bryce helped Lightning end Central Sparks' hopes of avoiding the play-off round in their bid to reach the final of the Rachael Heyhoe Flint Trophy.
Sparks were already sure of finishing in the top three but a win with a bonus point against Lightning could have seen them end as group-stage winners on net run rate depending on the outcome of the Southern Vipers-Northern Diamonds clash in Southampton.
But after Lightning posted a season-high 320 for 6 in their 50 overs after being asked to bat first, Sparks needed to overhaul that total inside 40 overs if they were to pick up the extra point, awarded if the winning team's scoring rate is 25 per cent better than their opponents.
Vipers, who came out on top at the Ageas Bowl, now go through directly to next Saturday's final at Northamptonshire's County Ground, while Sparks and Diamonds must play off on Wednesday to decide who joins them.
Kathryn Bryce shared a 207-run partnership for the second wicket with her younger sister, Sarah, the biggest in this season's competition among any of the eight teams.
Sarah was dropped on five but made the most of her good fortune, following her unbeaten 136 against Sparks last season with 90 off 104 balls, hitting 11 fours and a six before she fell to a fine one-handed catch by Issy Wong at mid-off.
Having completed her maiden List A hundred from 126 balls, Kathryn - who also captains Scotland Women - added another 61 from the next 30, racking up 21 fours and two sixes, batting for just short of three hours in total.
Lightning lost a wicket in the third over when Bethan Ellis was caught behind, Sarah Bryce having been put down at square leg in the previous over.
The Bryce sisters dominated for the next 36 overs before Sarah's demise, after which Abbey Freeborn, Beth Harmer and Ella Claridge all fell cheaply.
But Sonia Odedra helped Kathryn Bryce add 63 in the last five overs, including 35 from the final 12 deliveries from Wong (3-61) and Ria Fackrell before Bryce's magnificent effort ended when she was run out off the final ball of the innings.
Jones and Kelly - dropped on 24 off Odedra - gave the Sparks' chase the early momentum required, putting on 119 in 23.5 overs before the latter was superbly stumped off Yvonne Graves.
Jones was caught behind off England left-arm spinner Kirstie Gordon, who increased her tally to 16 as the competition's leading wicket-taker. Gwen Davies made 41 off 30 balls with four fours and two sixes but the pressure to score quickly inevitably cost Sparks wickets, Gordon, Odedra and Grace Ballinger picking up to each.