The 32-year-old has 25 wickets this season - 15 of them against Sussex - after he took five for 45 in Sussex’s 175. Northants were 91 for four at stumps.
Sanderson took ten wickets against Sussex earlier this month and his high-quality opening spell, when he took three for 17, helped reduce the hosts to 30 for four.
When Sanderson took his fourth wicket with the second ball after lunch Sussex were in dire straits on 67 for eight, but Jack Carson led a fightback with 52 - his maiden first-class half-century - as the last two wickets added 108.
Six of Sussex’s top eight are left-handers and they them struggled as Sanderson got the occasional ball to move away off a good length. The occasional delivery scuttled through too but there were runs to be had, as Carson and Henry Crocombe demonstrated in their ninth-wicket stand of 99.
Sanderson pinned Aaron Thomason (4) in his second over then removed Travis Head (1) and Stiaan van Zyl (5) in five balls with deliveries which deviated sharply off the seam. With Gareth Berg picking up his 300th first-class wicket when Tom Haines (1) was surprised by extra bounce, Sussex were in a mess.
Even when Sanderson and Berg rested, Northants kept the pressure on. Wayne Parnell had Ben Brown (9) caught behind off an inside edge and trapped Delray Rawlins (2) plumb in front. Tom Taylor knocked back Tom Clark’s (11) off stump and when George Garton (12) became Sanderson’s fourth victim Sussex were in a mess.
Carson and Crocombe had some luck – Carson was dropped on 26 and his partner on 13 and 32 – but they punished anything loose and relished playing in front of a crowd for the first time with 800 spectators admitted.
Carson hit five fours in a 99-ball half-century while Crocombe was unbeaten on 46 when Sanderson completed his five-for with the wicket of Jamie Atkins (5).
Sussex then hit back with the ball. Wicketkeeper Ben Brown ran 30 yards to take the catch when Ben Curran (7) top-edged a pull off Crocombe, Luke Procter (9) was caught behind after Atkins switched ends, Garton removed Ricardo Vasconcelos (20) to claim his 50th first-class wicket and Carson had Rob Keogh (24) caught at mid-wicket.
Carson, who made his maiden first-class 50, said: "There are three of us in the team who haven't played in front of a crowd before so even though it was only five or six hundred I was absolutely buzzing, especially when I got to my 50. I just wanted to take it all in, the applause of the crowd. It's the biggest crowd I've ever played in front of.
We did a pretty good job to take four wickets and the pitch is doing as bit so we're still very much in the game."