Lewis Gregory's seven wickets in Somerset's day-three haul of 15 put the visitors on the verge of victory in their LV= County Championship Division One match against Sussex at the BrightonandHoveJobs.com County Ground.
The 21-year-old seamer's first-innings return of 5-49 helped dismiss the hosts for 142 in reply to Somerset's 372 and, following on, Sussex were 162 for six - still 68 runs behind.
Gregory starred with the bat yesterday, clubbing a swift 47 at the end of the innings, but his contribution with the ball was even more decisive.
He decimated the Sussex middle order as they collapsed from 38 for one, with Luke Wells, who made an unbeaten 79, the only batsman to offer any resistance.
Gregory then added two more second-innings wickets in the final session of the day to leave Sussex, who had Ed Joyce unbeaten on 76, with an almighty mountain to climb to save the game.
Alfonso Thomas made the early breakthrough at the start of the day to remove Michael Yardy for 10, Sussex having started on 25 for one, but from then on Gregory took charge.
He trapped Rory Hamilton-Brown lbw for a duck, had Matt Machan caught by Marcus Trescothick, who was keeping wicket in place of the ill Craig Kieswetter, and bowled Ben Brown to leave the home team reeling on 61 for five.
Chris Jordan and James Anyon also fell to the Somerset youngster to leave Wells, who hit 11 fours in his 168-ball knock, fighting on his own.
Following on, and with Alex Barrow now behind the stumps for the visitors, Wells and Joyce got Sussex off to a steady start, sharing a partnership of 42 before Peter Trego dismissed Wells for 26.
This time it was Thomas who turned the screw by getting rid of Yardy and Hamilton-Brown to leave Sussex on 83 for three before Gregory again did for Machan and Brown.
Jamie Overton took the 15th wicket of the day, his third, to dismiss Ashar Zaidi and leave Somerset with just four more wickets to take on the final day.