Sussex 142 (Well 79*, Gregory 5-49) and 162 for 6 (Joyce 76*) trail Somerset 372 by 68 runs

Scorecard

Rory Hamilton-Brown was one of five wickets for Lewis Gregory Getty Images

Somerset's delight at Marcus Trescothick's restatement of his batting pedigree translated into an excellent bowling display on the third day at Hove as they forced Sussex to follow on under heavy skies and then rounded upon them once more with such conviction on a sunlit evening that they even had the glimmer of a three-day victory.

Ultimately, Sussex survived but it was a hollow achievement with only four wickets remaining, still 68 runs needed to make Somerset bat again and the weather set fair for the final day. Somerset's domination of this match has been striking, their tread optimistic again, assumptions that they will again struggle to retain their Division One status thrown into question.

Sussex's openers met the challenge with pragmatism, but twice a flimsy middle order failed to meet the challenge. Luke Wells batted through Sussex's first innings for 79 with stately deliberation - the first batsman to achieve the feat for Sussex since Richard Montgomerie 10 years ago - and it would be no surprise if Ed Joyce, 76 not out at the close, matched the feat in the second innings. If that was to happen, it would be unprecedented in Sussex's history.

Conditions have favoured the bowlers throughout, although far from impossibly, and with every wicket that fell - 15 in all for 279 runs - Trescothick's second-day century grew in status. He even undertook wicketkeeping duties in Sussex's first innings, which is not a great career move for a heavily built man with ankles he regularly needs to plunge into ice buckets, but the task was forced upon him by Craig Kieswetter's absence with a stomach ailment and he stopped what came his way with an air of contentment.

Alex Barrow, summoned from Taunton as an emergency wicketkeeping replacement, finally arrived three overs into Sussex's second innings, which even allowing for the regular loss of wickets suggested he may have travelled by tractor.

From his emergency vantage point, shifting a couple of yards from his usual post at second slip, Trescothick had a slightly straighter view of Lewis Gregory's threat. "A batsman who bowled" was how his captain labelled Gregory, fairly enough, after his 47 batting at No 11 in Somerset's first innings, but the lively medium pace of the Batsman Who Bowls was always threatening on this surface and he returned the second five-for in a first-class career barely 200 overs old.

Gregory is a zestful all-round cricketer, whose emergence this season, at 21, would help to offset Somerset's limited fast-bowling resources. A Devonian and product of Plympton in the Devon League, he brims with attacking intent and can now begin to build on the promise he displayed when he made his debut against Pakistan nearly four years ago.

Alfonso Thomas must wonder about the sense of playing too much Championship cricket at 37 when the decks at Taunton are unforgivingly true, but his appetite must also have been whetted by heavy morning cloud cover.

Jamie Overton, too, allied decent pace at times to a more acceptable degree of accuracy than is sometimes evident. He is a hulking figure for a 20-year-old, powerful rather than lithe, the sort of man who could cause a midsummer traffic jam in Barnstaple by strolling across the main road. England like their fast bowlers gigantic and he looks as if he must cost a fortune to feed. He needs protective handling and the mere sight of him vaguely feeling his back once or twice was enough to bring a spark of concern.

Mark Robinson, Sussex's director of cricket, judged that batting became a little easier after around 35 overs but, by then, both innings were in terminal decline as batsmen fell to a collection of indifferent strokes. Michael Yardy's footwork was leaden and Gregory uprooted Ben Brown's stumps twice in one day. In the second innings, Rory Hamilton-Brown slapped a slower ball that sat up in the pitch straight to the leaping Nick Compton at cover. Matt Machan was unhinged by a climbing delivery from Gregory in the first innings but his second-innings departure was a tale of self-harm as he flicked him off his hip to deep square leg.

Ashar Zaidi also faces a Sussex investigation into his response when he was adjudged caught at the wicket off Overton in the first innings. It was not his display of disbelief towards umpire Nigel Llong, as unwelcome as that was, as much as the fact that he took to Twitter to comment "stinker #wtf," so becoming the latest sportsman to fall foul of the social networking site.

"I didn't think he showed any dissent," Robinson said. "He was just desperately disappointed as he didn't hit the ball. The umpires are two of the best in the business but you get wrong decisions from time to time." He was more critical of the Twitter escapade, however. "You can't be doing that sort of thing whether or not it was a good or bad decision," he said. A fine and a reprimand are likely to follow.