James Fuller and James Harris produced an unbroken ninth-wicket stand of 149 as Middlesex took a grip of the match on the second day at Taunton.
The visitors had looked in trouble at 212-8 when Fuller walked out to join his fellow seamer but together they took the game away from Somerset.
By the close, Middlesex had moved to 361-8, a lead of 125, with Fuller unbeaten on a career-best 84 and Harris 69 not out.
Jim Allenby ended the day with 4-67, his best bowling figures for Somerset, but the loss of Jamie Overton to a back injury only three balls into his first over of the day proved a major blow for the hosts.
Middlesex lost wickets straight from the off, with Stevie Eskinazi (7) chipping the opening delivery from Craig Overton straight to James Hildreth at mid-wicket.
Dawid Malan (10) edged a delivery from Peter Trego onto his stumps, but Nick Gubbins reached a 72-ball half-century, with six fours and a six.
He fell for 67, caught at second slip by Marcus Trescothick off Allenby, who also had John Simpson caught behind for a duck and bowled James Franklin for five.
At lunch Middlesex were 146-6, but George Bailey cashed in after being dropped on 43, eventually making 71.
Still Somerset looked on top when Ollie Rayner and Bailey were both caught at slip but the pitch was to flatten out and Fuller and Harris took full advantage.
Neither gave a chance as their partnership reached three figures in 31 overs. The pair secured a fourth batting point, having established a Middlesex record for a ninth-wicket stand against Somerset, set back in 1895.
The home side had to settle for two bowling points on a green top and will need to bat long in their second innings to avoid a damaging defeat.
This post was last modified on 12/07/2016