England wicketkeeper Craig Kieswetter topped up the West Country county's overnight score to the tune of 73 in an impressive total of 517 in a must-win game in terms of Somerset's title aspirations.
Darren Pattinson bagged five for 95 for the visitors, but then watched as Nottinghamshire's top-order unravelled to 130-5 before Samit Patel, with 92, and his captain, Chris Read (75 not out), arrested the slide with an unbroken sixth-wicket stand worth 109 that helped cut the deficit to 239.
The Rose Bowl clash between Hampshire, who have reached 287-6, appears evenly poised as they hold a slender four-run advantage over Lancashire. A determined 72 from Jimmy Adams and 56 from Sean Ervine were the highlights of a solid Hampshire batting display.
In the Second Division, leafy New Road provided the backdrop for an intriguing mid-table tussle between Worcestershire and Northamptonshire. Having posted a first innings target of 287 Worcestershire's veteran opening bowlers Matt Mason and Alan Richardson took three wickets apiece to shoot out the visitors for 158. With a collective age of 71, they may be the oldest new-ball partnership in the land, but Mason with 3-34 and Richardson, who took 3-28, proved too wily for Northamptonshire.
Not that Worcestershire fared much better on a seamer-friendly pitch and by stumps they had stuttered to 72-8 in their second innings, with Lee Daggett taking the first four to fall in a stint of four for 25.
In the London derby at Lord's, Surrey appear to be heading towards a heavy three-day defeat to Middlesex. Already trailing by 256 on first innings, Surrey plunged from 75 without loss to 110-6. Middlesex seamer Toby Roland-Jones, 22, in his maiden season, continued to impress with 5-29, while Steve Davies posted a modest top score of 43 for the troubled Brown Caps.
Sussex continued their promotion push by dominating proceedings against Leicestershire at Grace Road. Opener Chris Nash posted a career-best 184 for the visitors and Leicestershire appear well adrift at 59-4 in their second innings.