Fresh from his match-winning 152 in the LV= County Championship success over Warwickshire on Saturday, Kieswetter scored 103 as Somerset overcame Durham's 147 with almost half their 40 overs to spare at Taunton.
The England one-day wicketkeeper plundered nine fours and six sixes in his 61-ball innings as last season's runners-up climbed off the bottom of Group B with their second successive victory in the competition. Somerset were also boosted by the return of Marcus Trescothick, three months after he ruptured ankle ligaments.
There were also big wins for Middlesex and Sussex.
Middlesex climbed to the top of Group A after thrashing Gloucestershire by 10 wickets thanks to impressive knocks from openers Chris Rogers and Dawid Malan.
Rogers scored an unbeaten 122 while Malan finished on 89 not out as Middlesex comfortably passed Gloucestershire's 214 for nine with almost 10 overs to spare.
The victory saw Middlesex steal a crucial march on their opponents in the race for a semi-final place, having started the day in second spot in Group A on 10 points, one more than third-placed Gloucestershire.
Sussex, meanwhile, cruised to an eight-wicket win over Northamptonshire at Arundel to consolidate their place at the top of Group C.
Chris Liddle took four for 21 and Scott Styris two for 12 as Northamptonshire were bowled out for 129 after James Middlebrook (37) and Rob Keogh (30) had rescued them from 46 for six at one stage.
Murray Goodwin then scored an unbeaten 67 and put on 96 for the third wicket with Ed Joyce (38 not out) as Sussex eased home in the 31st over.
Warwickshire kept up the pressure on Sussex with a 60-run victory over Unicorns at Edgbaston.
Darren Maddy struck an unbeaten 79 off 56 balls as Warwickshire made 228 for nine batting first and Chris Woakes (three for 30) then took three wickets in four deliveries as Unicorns were limited to 168 for seven in reply.
Also in Group C, Richard Johnson's maiden one-day half century spearheaded an impressive Derbyshire recovery that carried them to a three-wicket victory over Yorkshire with 14 balls to spare at Chesterfield.
The on-loan Warwickshire wicketkeeper survived two dropped catches to score 79 from 74 balls after Derbyshire had been 56 for four chasing Yorkshire's 238 for seven, which featured 51 from skipper Andrew Gale on his return from injury.
In Group B, Nottinghamshire maintained their push for a semi-final spot with a straightforward 88-run victory over Scotland at Trent Bridge.
Samit Patel led the way for the hosts with 82, while fellow England hopeful James Taylor contributed 68 and Adam Voges an unbeaten 64 as Nottinghamshire made 265 for five after winning the toss and batting first.
Patel then picked up two for 39 with his left-arm spin as Scotland were bowled out for 177, with fellow spinners Voges (three for 37) and Graeme White (three for 42) also among the wickets.
The win moves Nottinghamshire second in their group, one point behind leaders Hampshire, who have played a game more.
The Group A clash between Leicestershire and Lancashire at Grace Road saw the visitors claim a six-wicket win - their fifth triumph in seven games - which leaves them two points behind Middlesex and with three games still in hand.
Stephen Moore (60), Ashwell Prince (85) and Steven Croft all hit half centuries as Lancashire cruised to their victory target of 219 with 4.2 overs to spare.
The Foxes could only post a disappointing 218 for eight despite a brilliant unbeaten 83 from 18-year-old Shiv Thakor, while Lancashire's veteran seamer Glen Chapple claimed four for 46, his best return for six years in the 40 overs game.
In the County Championship, James Tomlinson emerged as Hampshire's hero as they collected a thrilling two-run victory over Essex in their LV= County Championship Division Two clash at Chelmsford.
The left-arm fast bowler took 5 for 86 including the wicket of Adam Wheater, who was last out for 98 as the hosts fell tantalisingly short of their 360-run target.
Wheater's swashbuckling innings spanning 111 deliveries contained four sixes and a dozen fours before he holed out to Sean Ervine on the long-on boundary.
Earlier Tomlinson seemed to have put the visitors in sight of a comfortable triumph when he roared in after lunch with three quick wickets.
But Wheater scored all 40 of a last-wicket partnership with Tom Craddock before finding Ervine's grateful hands with the victory in sight.
In the other game to finish on Sunday, centuries from Glamorgan skipper Mark Wallace and Kent's Sam Northeast ensured the two sides' Division Two clash ended in stalemate.
The hosts posted 168 for two in their second innings on a pedestrian final afternoon in Canterbury as former England Under-19 batsman Northeast carried his excellent limited-overs form of late into the four-day game by stroking 11 fours.
Wallace had earlier posted a 192-ball century and featured in a ninth-wicket stand with Dean Cosker worth 103 in 19 overs that denied Kent their chance to enforce the follow-on.