Middlesex 250 for 7 (du Plooy 66*, Robson 58, Sanderson 5-58) lead Northamptonshire 207 (Gay 42, Sanderson 40, Roland-Jones 5-49) by 43 runs
Half-centuries from Sam Robson and Leus du Plooy gave Middlesex the upper hand on the second day of their Vitality County Championship game against Northamptonshire at Merchant Taylors' School.
Robson enhanced his impressive record at the Northwood venue, which includes four first-class centuries, by grinding out a vital 58 while Du Plooy hit an unbeaten 66 as the home side reached 250 for seven, a first-innings advantage of 43.
However, Ben Sanderson kept Northamptonshire right in contention with figures of five for 58, regaining his status as Division Two's leading wicket-taker after being briefly displaced by Middlesex captain Toby Roland-Jones.
Northamptonshire were dismissed for 207, with Roland-Jones taking a season's best of five for 49 - and the visitors' hopes of restricting Middlesex to a lower total were not helped as Prithvi Shaw shelled a trio of slip catches.
Sanderson and Lewis McManus, who had hauled Northamptonshire out of difficulties the previous evening, did enough to usher the visitors beyond the key landmark of 200 as they extended their partnership to 83.
Sanderson unfurled a pair of classic drives to the boundary off Tom Helm and thoughts of a maiden first-class half-century must have entered the veteran seamer's mind as he overtook McManus to reach 40.
However, those thoughts were dashed when Henry Brookes bowled Sanderson around his legs and, although debutant Dom Leech cracked a cover boundary to raise the visitors' 200, Roland-Jones quickly wrapped up the innings by capturing their last two wickets in three balls.
In reply, Middlesex's opening pair both survived close calls during the hour prior to lunch, although they made it to the interval unscathed on a surface with variable bounce and pace.
With just a single to his name, Robson edged a rising delivery from Justin Broad through the slips, while Mark Stoneman offered a tricky slip chance off the same bowler and Shaw, going low to his left, was unable to hang on.
Sanderson made the breakthrough soon after the resumption, getting the ball to swing and uprooting Stoneman's off stump for 36, but Robson and Max Holden dug in for an afternoon of laborious progress.
Robson cut the seamers with authority to keep the scoreboard moving and passed 50 from 118 balls with a sweet cover drive for four off leg-spinner Yuzvendra Chahal, whose change of ends then bore immediate fruit as he trapped Holden leg before.
Northamptonshire might have removed Robson as well in the next over, with Shaw - who had also put down Holden - fumbling another opportunity, but Sanderson made amends immediately after tea with two wickets in as many deliveries.
With Robson lbw to one that kept low and Stephen Eskinazi succumbing in identical fashion, Middlesex were suddenly wobbling at 129 for four but a bristling partnership of 72 between Du Plooy and Fernandes was exactly what they needed.
Leech eventually brought the stand to an end with his first Northamptonshire wicket, having Fernandes taken at second slip, but Du Plooy remained to nudge his side into the lead with a crisp off-driven boundary.
However, Sanderson returned with the new ball and promptly claimed two more wickets in quick succession, completing his third five-for of the summer before Roland-Jones launched a late flurry of boundary-hitting.
He took two fours from successive Sanderson deliveries and had just cracked Broad to the fence to earn Middlesex a batting bonus point when the deteriorating light brought play to a close.
