Glamorgan 199 and 186 for 5 (Carlson 54*, Roland-Jones 2-50) trail Middlesex 470 for 9 dec (Holden 107, du Plooy 89, Geddes 83, Kellaway 4-61) by 85 runs
Toby Roland-Jones struck twice in as many deliveries to boost Middlesex's push for a first Rothesay County Championship victory of the season against Glamorgan at Lord's.
Trailing by 271, the visitors responded with an opening stand of 93 between Zain ul Hassan and Eddie Byrom, but were pegged back to 148 for 5 when Roland-Jones bowled Colin Ingram and Ben Kellaway with successive balls.
However, Kiran Carlson's defiant unbeaten 54 maintained Glamorgan's hopes of staving off defeat, guiding his side to 186 for five - a deficit of 85 - at stumps.
Middlesex had earlier declared on 470 for 9, although both Leus du Plooy and Ben Geddes missed out on centuries, falling to Kellaway for 89 and 83 respectively as the Glamorgan off-spinner finished with 4 for 61.
Du Plooy, who resumed 15 short of a first ton of the campaign, added just another four to his overnight tally before being undone by a ball from Kellaway that pitched and turned to clip his off stump.
Geddes was the more fluent of the pair towards the end of their fifth-wicket stand of 131, slashing Timm van der Gugten twice to the third man fence as he advanced to within sight of his maiden century in Middlesex colours.
But Geddes became stuck in a rut, surrendering his wicket to Kellaway with a loose drive that sailed straight to short midwicket before Jack Davies, slog-sweeping the spinner, picked out Byrom diving to his left.
Roland-Jones restored some momentum to the innings with a pugnacious 31 from 27 balls, launching Kellaway for a straight six and clubbing Ned Leonard for a trio of boundaries in his first over of the morning.
Leonard had the final say by collecting Roland-Jones' wicket, with Chris Cooke soaring to his right for an outstanding one-handed catch off the outside edge, then adding a stumping to dismiss Zafar Gohar in Kellaway's next over.
However, there was to be no five-wicket haul for Kellaway, bowling unchanged from the Nursery End, as Middlesex declared following a few lively blows by Dane Paterson in order to put their opponents in again before lunch.
While the seamers beat the bat several times on either side of the interval, they could not find a way through - although there was a let-off for Byrom with five to his name, steering Roland-Jones to second slip only for the chance to go begging.
Byrom capitalised, dispatching successive Ryan Higgins deliveries to the leg-side rope and pummelling a Henry Brookes half-volley through the covers for four more while his partner also found ways to keep the scoreboard moving along.
With half-centuries firmly in their sights, though, the opening pair departed in successive overs as Byrom top-edged a sweep off Gohar to backward square and Higgins, who had just switched ends, had ul-Hassan caught behind.
That double success galvanised the home side, who struck again shortly after tea when Paterson nipped a ball away to pin Sam Northeast leg before - although height could well have saved the Glamorgan captain on another day.
Paterson's swing continued to cause problems for the batters, with Ingram very much in survival mode and Carlson edging a ball that dropped fractionally short of slip's outstretched hands.
But the Glamorgan pair added 40 before Roland-Jones brought himself back on to break the partnership, knocking back Ingram's off stump and then dismissing Kellaway, who played on first ball.
Carlson continued the battle, bringing up his half-century from 84 deliveries, while Cooke remained alongside him on 16 when deteriorating light prompted the umpires to halt play five overs before the scheduled close.
