The Bears raced past the home team’s modest 145 for eight with 31 balls to spare and eight wickets intact.
De Lange bowled his fellow South African with the penultimate delivery of the 18th over and then, with Somerset needing just eight to win from the last over, restricted them to a miserly six.
They duly restricted the chase to 134 for eight, Harry Gurney closing out an 11-run victory with a fine last over after Dan Christian took three middle-order scalps.
Birmingham Bears remained on course for the knockout stage with a regulation defeat of rock-bottom Durham.
Nottinghamshire, meanwhile, looked beatable at the halfway point after posting 145 all out at New Road.