Southern Vipers began their defence of the Rachael Heyhoe Flint Trophy with a four-wicket victory over Lightning.
England’s Danni Wyatt hit 53 as Vipers, who won the inaugural trophy in 2020, remained unbeaten in the competition.
Sophia Dunkley’s unbeaten 104 and Tash Farrant’s five-wicket haul guided South East Stars to victory over Sunrisers.
Heather Knight and Anya Shrubsole led Western Storm to a last-ball win over Thunder while Northern Diamonds fell to Central Sparks in another close affair.
Vipers won all six group games of the eight-team, 50-over competition in 2020, before defeating Diamonds in the final – and they continued in that fashion with a comfortable opening-day victory.
The tournament – featuring the eight England and Wales Cricket Board regional hubs – was set up after the coronavirus pandemic thwarted last year’s planned introduction of The Hundred.
Teams will play each other once during the league stage, with the side who top the regular-season table advancing direct to the final on 25 September.
The second and third-placed teams will contest a play-off on 22 September, with the winner also progressing to the final.
England stars shine on a day for the batters
While Wyatt top-scored for Vipers at the Ageas Bowl in Southampton, it was very much a collective effort from the reigning champions that enabled them to subdue East Midlands outfit Lightning.
Wyatt’s England team-mate Tammy Beaumont struck 57 as Lightning recovered from 43-4 to post a competitive 214-9.
However, Vipers knocked that off with more than three overs to spare as Wyatt set the platform for Paige Schofield (38no) to see the hosts home at 215-6 despite fine bowling from Kirstie Gordon (4-35).
Sunrisers, by contrast, are still seeking their first trophy win after losing all six group games last summer and they were well beaten by 135 runs at home to South East Stars.
Dunkley’s ton, aided by 59 for England team-mate Bryony Smith and Alice Davidson-Richards’ 53, helped Stars amass 324-7 before Farrant took 5-33 as Sunrisers were dismissed for 189 in reply at Chelmsford.
The main drama came in Saturday’s other two games, chiefly at Bristol where Western Storm outlasted northern-based Thunder in a thriller.
Emma Lamb (121) and Piepa Cleary (79) shared a superb 177 partnership as Thunder posted 291-8 but England skipper Knight and fellow stalwart Shrubsole led a fantastic fightback from the hosts.
Opener Knight’s 91 gave Storm a chance but all appeared lost when they slipped to 251-9 only for Shrubsole to hit an unbeaten 61 off 41 balls to pinch victory by hitting a boundary off the last ball.
It was almost as close at Emerald Headingley where England wicketkeeper-batter Lauren Winfield-Hill’s 110 steered last year’s runners-up Diamonds to 279, despite Issy Wong taking 5-49 for the visitors.
Central Sparks slipped to 6-2 in reply but Amy Jones hit a run-a-ball 114 and Sarah Glenn smashed six sixes in an unbeaten 71 off 52 balls to give the Midlanders victory with 10 balls to spare.
Results and scorecards
Southern Vipers (215-6) beat Lightning (214-9) by four wickets (Southampton) – scorecard
Sunrisers (189) lost to South East Stars (324-7) by 135 runs (Chelmsford) – scorecard
Western Storm (295-9) beat Thunder (291-8) by one wicket (Bristol) – scorecard
Northern Diamonds (279) lost to Central Sparks (283-8) by two wickets (Headingley) – scorecard