Somerset 122 for 9 (McKerr 3-20) trail Surrey 485 (Elgar 110, Stoneman 85, Burns 78, Roy 63) by 363 runs
Surrey moved into a position to complete their eleventh County Championship victory of an outstanding season on the second day of the match with Somerset at Taunton.
Having extended their first innings from an overnight 368 for 4 to 485 all out, Dean Elgar making 110 against his former county, the newly crowned champions then reduced their hosts to 122 for 9 before rain and bad light intervened to wipe out the final session.
Conor McKerr claimed 3 for 20 as Somerset's batting line-up self-destructed for the third successive game to leave them still needing points to guarantee runners-up spot.
Only 19-year-old Tom Banton, opening the batting on his first-class debut, showed much resolve. He was eighth man out for 30, while tail-ender Josh Davey was 22 not out at stumps.
The day began with Somerset needing two wickets in 14 overs to claim a second bowling point. Their chances suffered a blow when former Surrey wicketkeeper Steve Davies dropped Elgar on 84, Davey being the unlucky bowler.
Tim Groenewald produced a good delivery to bowl Ben Foakes for 25, but at the end of the 110th over the scoreboard read 415 for 5 and, while Surrey claimed five bonus points from the innings, Somerset had to settle for one.
Elgar went to a century off 170 balls, with 17 fours, and at one point the visitors were 446 for 5. Wickets then fell in a cluster, Tom Abell claiming three and Jack Leach two, despite a late assault from Amar Virdi, whose four boundaries off successive balls from Leach took him to a career-best 21 not out.
Lunch was taken at the end of the innings and Somerset's reply got off to a bad start when Marcus Trescothick edged Morne Morkel's first ball to second slip.
The home side had been dismissed cheaply in their previous four Championship innings and were soon 11 for 3 as Morkel finished a five-over opening spell with figures of 2 for 8.
Tom Curran pinned James Hildreth lbw before McKerr got to work from the Somerset Pavilion End, sending back Davies and the Overton twins as the hosts slumped to 53 for 7. Abell was caught behind down the leg side off Rikki Clarke for 21, having played better than most.
Banton had battled away for 62 balls, putting more experienced partners to shame, before top-edging an attempted sweep off Virdi and falling to a second diving catch of the innings by wicketkeeper Foakes.
Groenewald helped Davey add 25, but then fell leg-before to Clarke for 11 with the total on 109 and his side still needing a further 227 to avoid following on. That was 214 at the close and only more bad weather would appear to stand between Surrey and a second crushing Championship victory over Somerset this summer.
Davey looked relatively untroubled in moving to 22, an indication that poor batting, rather than any terrors in the pitch had been responsible for Somerset's plight.
Surrey's bowlers maintained a better line and length than their opponents had on day one, but were aided by some abysmal shots.