Nottinghamshire and Somerset's batsmen continued to excel in friendly conditions after learning that their Division One status had been preserved.
Both sides would still have been vulnerable if Derbyshire had been able to win their final contest but, after their defeat to Warwickshire, Somerset reached 372 all out in their first innings before Nottinghamshire ended the day on 139-3, a lead of 173.
James Hildreth was the star man for the visitors at Trent Bridge, eventually falling for 161, while Nottinghamshire's Harry Gurney finished with career-best figures of 5-81.
Hildreth passed 1,000 first-class runs for the season during his stand of 248 with Nick Compton, who was eventually bowled by Samit Patel's quicker ball for 87.
Gurney, who had removed Marcus Trescothick the previous evening, boosted his own tally of wickets after the second new ball had been taken.
Its first victim had been Hildreth, caught behind off Luke Fletcher - presenting Chris Read with his 750th catch for Nottinghamshire and a dismissal that triggered three wickets in a seven-ball spell.
Jos Buttler, rumoured to be playing his last match for Somerset, was trapped lbw by Gurney for 13 before the left-arm fast bowler also removed Craig Meschede (0).
Lewis Gregory chanced his arm in making an undefeated 40 which included an uncontrolled pull for six off Brett Hutton, but then ran out of partners as the innings was wound up.
Steven Mullaney set the tone for Nottinghamshire's second innings, racing to a half-century in just 38 balls before driving Peter Trego to mid-off.
Wessels and Michael Lumb each added 20 but the day had a slightly curious ending as Craig Kieswetter removed the wicketkeeper's pads and gloves to have a bowl with Trescothick - rather than Buttler -- putting them on for the final five overs.
With both sides locked together on exactly the same number of points - Somerset having won one game more this season - Nottinghamshire will need to force the win on the final day of the season to finish the higher of the two.