At stumps Sussex were 120 for 6 in reply and had largely squandered the good work of their bowlers in the opening two sessions.
SEE ALSO Laurie Evans guides Sussex Sharks to first T20 finals day since 2012 | Sussex back in promotion spots after 243-run victory | Lizelle Lee's century sees Surrey Stars beat Loughborough Lightning in Kia Super League finalThere was heavy cloud cover for much of the day and the floodlights were on and off. Both attacks exploited the conditions well, although batsmen on both sides were guilty of some loose shots as well.
Only Middlesex’s Max Holden made a half-century, which he reached the ball before Middlesex were bowled out just before tea.
Sussex began their reply positively and openers Luke Wells and Phil Salt had 45 on the board in 7.5 overs. But when Wells (24) drove Ethan Barber uppishly to mid-off it sparked a collapse which saw four wickets fall for 47 runs.
Salt (32) squandered a good start when his airy drive was well caught by Dawid Malan running back from cover. Tom Haines (15) was trapped on the crease and edged to second slip, Harry Finch (17) bowled by one that kept low and Luke Wright leg before to James Harris for a fourth-ball duck.
Chris Jordan and Ben Brown saw out the remainder of the day which ended six overs early because of bad light.
Sussex would be happy with any sort of lead and they will be confident that their bowlers can inflict similar damage in Middlesex’s second innings in a game the hosts really need to win if they are to mount a late promotion charge.
Their four-man seam attack dovetailed effectively and the wickets were shared around but the most impressive bowling came from Jordan, who took 3 for 17 in his second spell after lunch including Eoin Morgan, who was caught at slip after battling for 90 minutes and facing 77 balls for his six runs. At one stage England’s one-day captain went 35 balls without scoring.
Jofra Archer finished with 3 for 34, ending the innings when he uprooted Steve Finn’s leg stump the delivery after Holden reached a battling 104-ball half-century off 155 minutes with five fours. The next highest scorer was 39 extras.
Middlesex never recovered after losing four top-order wickets in the morning session for 30 runs. Ollie Robinson straightened one down the slope to pin Sam Robson (8) and in the next over Steve Eskanzi was beaten for pace by Jofra Archer for a duck.
David Wiese struck with his third ball when Gubbins chased a ball he could have ignored and edged to second slip for 29. The South African then nipped one back off the seam to trap Malan in front for six.
The ball has done a lot all day because of the overhead conditions and there's a bit of uneven bounce from the Nursery End so we're pretty pleased with how the day ended up.
We possibly went searching for wickets early on but the bowlers adjusted their lengths and we built up pressure and got our rewards.
I am really pleased with my own batting at the moment. Today I was able to assess the situation, knuckle down and be patient and wait for balls that were in my strong areas to hit and that suits my game. Hopefully we can wrap their innings up in the morning then put ourselves in a strong position in the third innings, which is something we've done well in the last couple of Championship games.
It did a lot all day and in the morning when we took four wickets we could have taken even more. It swung, there was some seam movement as well and I thought our bowlers exploited the conditions really well.
We were pleased with the score we restricted them to but we knew it would be tough for us as well, even though it was a shorter session after tea to bat.
But we bat all the way down to 11 and we're going to need every run. If we can get a lead we can set the game up but there is a lot of hard work ahead in the morning.