Somerset 298 for 5 (Trego 101, Groenewald 57) beat Warwickshire 181 (Evans 50, Thomas 3-30, Groenewald 3-37) by 117 runs
Former Warwickshire allrounder Groenewald blasted 57 from 35 balls towards a total of 298 for 5 and then took 3 for 37, including two wickets in three deliveries when the home side were ahead of Somerset's initial scoring rate.
Freddie Coleman, with 25 in his second first-team game of the season, and Jonathan Trott, with 29 after bowling eight overs for 35 runs, made some headway and Somerset needed a breakthrough when Groenewald had Tim Ambrose caught behind for 33.
Although Laurie Evans went on to make 50 before driving to cover off Alfonso Thomas, the innings folded for 181 after two wickets in four balls for Lewis Gregory.
It was probably a match too far for Warwickshire after they overcame injuries and international call-ups to win four in a row to reach finals day in the NatWest T20 Blast.
The team that picked up the threads of the 50-over competition was further weakened. Boyd Rankin is now away with England Lions and Varun Chopra sustained a thigh injury while making 86 not out in the T20 quarter-final against Essex.
Rankin's absence left Warwickshire without a full complement of four front-line seam bowlers and Chopra's setback meant that William Porterfield became the fourth player to captain the team this season.
Having led Ireland for a number of years, Porterfield is well versed in the job and with rain around - the start was delayed by 15 minutes - he followed protocol by choosing to field first.
Rikki Clarke and Oliver Hannon-Dalby conceded only 30 runs in 11 overs and claimed a key wicket when Nick Compton had his off stump knocked out by Clarke after making 12.
Marcus Trescothick then began to impose himself, clearing the extra cover rope with ease off Recordo Gordon, and it was a stroke of luck, as well as a good recovery after a mishap in the field, that brought about his dismissal for 41.
Gordon went sprawling as he set off to chase Trego's dab towards third man and with the batsmen lured into going for a third run, Trescothick was beaten by Gordon's accurate return to wicketkeeper Tim Ambrose.
However this was as good as it got for Warwickshire. Trego, after a let-off when dropped at square leg on 29, took control with three sixes in a first 50 from 62 balls and then accelerated to his 100 in the next 39 deliveries.
South African Colin Ingram chipped in with 47 in a stand of 92 in 16 overs before a ball from Gordon got through to his leg stump and Trego, who finished with six fours and four sixes, was making room to cut when bowled by Jeetan Patel for 101.
In the latter part of his innings, Trego was upstaged as Groenewald contributed 53 to a stand of 87 in 10 overs before Patel struck. In all Groenewald smashed seven fours and a six against the first of his three counties until he was caught behind off Gordon for 57.
A second defeat in four group games is a blow for Warwickshire, but not terminal, as they strive to stay in contention in all three competitions.