Surrey 158 for 4 (Finch 102*) beat Somerset 157 for 9 (Banton 47, Tahir 4-25) by six wickets
Aaron Finch clattered a remarkable nine sixes in a brilliant 102 not out from 53 balls at the Kia Oval as Surrey kept alive their slim hopes of reaching a Vitality Blast quarter-final by severely denting Somerset's qualification ambitions with a barnstorming six-wicket victory.
Chasing Somerset's 157 for 9, in which openers Tom Banton and Babar Azam put on 80 in 8.2 overs before the innings fell away dramatically against the spin of Imran Tahir and Gareth Batty, Finch's assault emphatically settled matters and brought Surrey their fifth win of the campaign.
They now have 12 points and Somerset, in fourth place with 13, need to beat Middlesex in their final group game on Friday if they are to have a chance of making the last eight.
Tahir's 4 for 25 from four overs, each wicket wildly celebrated as is the South African leg spinner's wont, and Batty's 2 for 24, were as important to Surrey's cause as Finch's power-hitting. The two Surrey spinners totally changed the impetus of the Somerset innings, after they had been given a flyer by Banton and Babar after choosing to bat first in front of a 25,000 sell out crowd - Surrey's fourth full house in six home games so far.
But Somerset might still have defended their eventual total if Craig Overton had managed to cling on to a boundary-edge catch in the eighth over, with Finch on 33, when the Australian pulled a short ball from leg spinner Max Waller to deep mid wicket and saw the ball bounce through Overton's upstretched hands and go over the rope for his fourth six.
Finch did not give any other chances, however, as he followed up that blow, two earlier straight sixes off Tim Groenewald and an opening pulled maximum off Overton, with further sixes off Roelof van der Merwe (both in the ninth over), Waller (spectacularly reverse-driven over long off in the tenth over) and two off Tom Lammonby's left arm seamers in the 11th over.
By then, it hardly mattered that Sam Curran had skied Van der Merwe's left arm spin to Tom Abell in the ring, having contributed just 9 to a second-wicket stand of 61 in a mere 5.1 overs, following Mark Stoneman's departure for 18 to the last ball of the sixth over, or that Ollie Pope was run out for 7 in the 15th over.
Finch completed his 52-ball hundred, his seventh in T20 cricket, in the same over with his fifth four - whipped wide of mid on off Groenewald, and Will Jacks smacked two successive fours before being caught and bowled for 9 by Van der Merwe with just eight more runs wanted. Jordan Clark then hit Jerome Taylor into the pavilion for the evening's final six as victory arrived with 3.3 overs to spare.
Somerset, earlier, were flying along at ten runs per over until the two veteran Surrey spinners began to exert their stranglehold.
Banton's 28-ball 47 was by far the most fluent of the Somerset batting efforts, with legside sixes off Curran and Clark and five fours besides - the pick of them a reverse flip for four off Tahir in the game's very first over. Babar hit only three fours, but played his part too in an opening partnership which hinted at an eventual Somerset total of around 200.
Once Banton had fallen in the ninth over, however, the innings lost all its forward momentum and soon James Hildreth followed, stumped off Batty in the next over for 1 when he advanced to a well-flighted delivery outside off stump and yorked himself.
Worse was to come for Somerset, though, with Babar chipping a return catch to Tahir to go for a 31-ball 37 and, in the same 11th over, Ed Byrom also giving the South African leg-spinner a simple caught and bowled off a looping leading edge.
Four wickets had gone in 17 balls, for the addition of only 13 runs, but from 93 for 4 a mini-recovery was quickly stifled as Batty removed Abell for 14, stumped as he attempted a reverse sweep and Tahir had Van der Merwe leg before for 2.
That was 114 for 6, in the 15th over, and from there, the lower order did its best to get Somerset up to a defendable total with Lammonby reaching 19 before being caught behind in a nine-ball Jade Dernbach over, the 18th, costing 12 runs.
Clark and Curran grabbed the wickets of Overton and Groenewald in the closing overs and Somerset needed their tenth wicket pair of Taylor and Waller to make sure they batted through their 20-over allocation.