Matt Machan's maiden first-class century helped Sussex set up the prospect of victory within three days in the LV= County Championship Division One encounter with Somerset at Taunton.
Unbeaten on 58 overnight, the 22-year-old was finally dismissed for 103, having extended his third-wicket stand with Michael Yardy, who hit 156, to 275 in 52 overs before the visitors were bowled out for 363.
That meant a first-innings advantage of 119 and, after Marcus Trescothick had been dismissed for a fourth successive duck by James Anyon on his way to 4-56, Somerset struggled to 197 for seven in their second innings, a lead of 78
Yardy began the day on 94 after benefiting from some generous bowling the previous evening and soon moved to his hundred from 99 balls, with a pull for four off Jamie Overton.
Steve Kirby and Overton bowled without much luck early on. Machan then greeted the introduction of off-spinner Alviro Petersen with a six over long-on before being dropped on 84 by James Hildreth at cover off the same bowler.
That was all the luck the Brighton-born left-hander needed to follow Yardy to three figures, reaching the milestone with a back-foot shot through midwicket of Craig Meschede. He had faced 138 balls, striking 13 fours and a six.
The brilliant stand with Yardy was put into perspective, not only by Somerset's first day difficulties on a lively pitch, but the Sussex batting that followed.
Their last eight wickets added only 80, with Yardy playing a delivery from Overton onto his stumps having hit 27 fours in a powerful 177-ball innings.
Having bowled themselves back into contention, Somerset needed a good start to harbour hopes of turning the game. Instead, the third ball was a wonderful delivery from Anyon, which Trescothick edged through to wicketkeeper Ben Brown.
The former England opener could hardly believe his luck after two Twenty20 ducks in as many games and a first innings nought in this one. Walking back to the pavilion he took off his helmet and raised his bat to the crowd, suggesting his sense of humour remains intact.
Nick Compton also failed for the second time in the match as Anyon and Steve Magoffin continued to get more out of the wicket than their Somerset counterparts.
Petersen, Hildreth and Jos Buttler (35) could not turn promising scores into half-centuries, while Craig Kieswetter - who batted more responsibly than most in the first innings - fell just before stumps for 37.
Machan and Meschede are expected to leave the match tomorrow, having been nominated to drop out when Sussex skipper Ed Joyce and Somerset spinner George Dockrell return from international duty with Ireland.