Surrey moved up to second in the South Group of the NatWest T20 Blast by holding their nerve to beat Somerset by three wickets in a low-scoring thriller at the Kia Oval.
Put in to bat, Somerset lost their way after a productive third-wicket stand of 46 in five overs between Peter Trego and Colin Ingram to post a modest 136 for nine, with Matt Dunn claiming 3-8 off two overs at the end.
Surrey overcame the early loss of Jason Roy thanks mainly to Kevin Pietersen, who top-scored with 39 off 28 balls, and Tillakaratne Dilshan’s 29.
Just as Somerset had found earlier, runs then became harder to come by, but Chris Tremlett and Zafar Ansari eventually saw the hosts over the line with five balls to spare to the delight of a 20,000 crowd.
Marcus Trescothick, who kept wicket for Somerset in the absence of the injured Craig Kieswetter, fell early to Tremlett and Dunn had Nick Compton taken at cover to make it 26 for two.
Trego raced to 20 off 10 balls, pulling Tremlett to the fence before driving the England paceman down the ground for four and lifting Azhar Mahmood over square-leg for a maximum.
But Ingram was stumped off Ansari, Trego holed out to Gareth Batty at long-off for 30 and Craig Meschede was trapped leg before as Somerset slipped to 86 for five on a pitch offering plenty of assistance to Surrey's slow bowlers.
James Hildreth swept and pulled Batty and Ansari for four in a 35-run stand with Craig Overton, but thereafter the visitors lost all momentum.
Overton was lbw swinging across the line to Ansari, who was lying in wait when Hildreth attempted to lift Dunn over cover for six.
Dunn found himself on a hat-trick when Tim Groenewald was beaten for pace before the returning Azhar broke through Max Waller's defences.
In reply, Surrey lost Roy early on, bowled by Dirk Nannes for four as he tried to whip a full-length delivery through midwicket.
After being struck on the helmet by Alfonso Thomas, Dilshan hooked Nannes for six – only to flick the next ball straight to short midwicket.
That brought the curtain down on a second-wicket stand of 66 in eight overs with Pietersen, who then fell for 39 when he was bowled by leg-spinner Waller trying to cut.
Surrey went eight overs without finding the boundary thereafter, during which Gary Wilson, sweeping Waller, looped a catch to short fine-leg,
Steven Davies was caught at midwicket off Groenewald and Robin Peterson was the victim of a horrendous run-out.
With 22 needed off three overs, Azhar hit a four back over the head of Overton – who was then smeared through cover by Tremlett.
Azhar was brilliantly caught on the boundary at wide long-off in the penultimate over, but Ansari clipped the first ball of the final over to deep fine-leg to seal an important victory for his side.
Surrey skipper Wilson said: "We limped over the line at the end, but I had full confidence in Zafar Ansari.
"I thought we were brilliant in the field and with the ball. It was brilliant having four spinners, taking the pace off the ball.
“Dilly (Tillakaratne Dilshan) usually takes the first over and it wasn't his night tonight, but the way Gareth Batty, Robin Peterson and Zafar bowled was brilliant and Matt Dunn was superb at the end."
Somerset captain Thomas added: "I thought we were about 30 runs short. Psychologically, 150 was always going to be a good score on that wicket, but we just couldn't build a partnership and it was difficult with their spinners dominating the middle period."