Gloucestershire 86 for 0 (Dent 45*, Harris 30*) trail Surrey 603 (Smith 234*, Clark 137, Pope 84, Curran 64) by 517 runs
Jamie Smith's maiden double-century and a second first-class hundred from Jordan Clark gave Surrey a stranglehold on the LV= Insurance County Championship match with Gloucestershire at the Seat Unique Stadium, Bristol.
Unbeaten on 111 overnight, Smith had moved to 234 not out, from 430 balls, with 34 fours, by the time his side were finally bowled out for 603 shortly after tea on day two.
It was a phenomenal effort of concentration from the 21-year-old, who occupied the crease for nine hours and 28 minutes, sharing a stand of 244 - a record for Surrey's eighth wicket - with Clark to stifle the life out of a willing, but uninspired home attack.
Clark provided positive support, making 137 after walking out at 349 for 7, with Surrey just having failed to claim a fourth batting point. He struck 15 fours and 2 sixes in an imposing 177-ball innings.
By the close, Gloucestershire had replied impressively by posting 86 without loss. Openers Chris Dent, who was 45 not out, and Marcus Harris, unbeaten on 30, produced an array of positive shots in the face of considerable scoreboard pressure.
The day began promisingly for the hosts as from 294 for 4 Surrey lost three wickets for the addition of 55 runs. Sam Curran was first to go, having added seven to his overnight 57 before being caught at gully driving a ball from Ryan Higgins.
The next delivery saw Colin de Grandhomme bowled by a Higgins no-ball. He was then dropped at slip off the luckless Mohammad Amir before departing for 11, leg-before to Matt Taylor.
When Will Jacks was caught and bowled by David Payne off a top-edged pull shot for 13, Gloucestershire were well in the game.
But by lunch, Smith and Clark had added 37 to take the total to 396 for 7, Smith having gone past his previous career-best score of 138, and the afternoon session saw both batters dominate.
Smith went to 150 with a leg-side boundary off Payne and in the same over produced one of the shots of his innings, a glorious straight drive for four that exuded quality.
Clark brought up his fifty with an off-driven boundary off Amir, the one Gloucestershire bowler to threaten repeatedly, albeit in vain as he finished wicketless.
Smith's double hundred was brought up with a flourishing cover drive for four off Miles Hammond, his 30th boundary. It was greeted with warm applause and cheers from the Surrey balcony.
Clark took two runs through the off-side from the off-spinner to reach his hundred off 151 balls, the second fifty having occupied only 60 deliveries.
By tea, which was taken at 574 for 7, the partnership had overtaken the previous highest eighth-wicket stand for Surrey in first class cricket, an unbroken 222 between Ben Foakes and Gareth Batty against Hampshire at the Ageas Bowl in 2016.
Still captain Rory Burns wanted more runs on the board and there were five more overs of toil for the home bowlers.
Some lusty swings accounted for Clark and the tail-enders, Dan Worrall falling to Jack Taylor, who has reinvented himself as a leg-spinner having had his off-spin action deemed illegal back in 2017.
Surrey's seamers found it as tough as their Gloucestershire counterparts to make inroads on the slow pitch as Dent and Harris confidently saw out 24 overs to the close.
