Sussex 442 (Simpson 183*, McAndrew 53 Holland 4-64) and 66 for 2 lead Leicestershire 275 (Handscomb 92, Mulder 53, Hunt 4-70) by 233 runs
Peter Handscomb continued his fine form with 92 but Leicestershire are up against it against second division leaders Sussex at Hove.
The Australian took his tally to 640 runs in this season's Vitality County Championship, but the visitors lost their last four wickets for 18 runs after Handscomb was seventh out with the score on 257.
Left-armer Sean Hunt profited handsomely after switching to bowl down the slope at the 1st Central County Ground, picking up Handscomb, Ben Mike, Scott Currie and Josh Hull in 11 deliveries as Leicestershire were bowled out for 275.
It gave Sussex a lead of 167 but they elected not to enforce the follow-on and in 15 overs before stumps they lost Oli Carter - squared up by Ian Holland - and nightwatchmer Jack Carson, closing on 66 for 2 - a lead of 233.
The second day had started encouragingly for the Foxes after they took Sussex's last three first-innings wickets for 11 runs in 23 balls with skipper John Simpson finishing unbeaten on 183, although he was only able to add three runs to his overnight score in a total of 442.
Holland finished with 4 for 64 after claiming last man Hunt but his day took a turn for the worse when Ollie Robinson removed him during a high-class new-ball spell of 3 for 23 from eight overs by the England pace bowler.
Bowling a fullish length, Robinson found enough movement to find Holland's edge in his second over. Lewis Hill and Rishi Patel took advantage of some wayward bowling by Hunt before 45 for 1 quickly became 51 for 4. Hill was leg before trying to work Robinson through the leg side and Robinson struck again when he found some extra bounce and Lewis Goldsworthy gave Tom Alsop the first of four catches at slip. In between, Nathan McAndrew picked up the important wicket of Patel who lost his off stump shouldering arms to an in-ducker.
Handscomb and Wiaan Mulder rebuilt the innings during a hot afternoon, adding 108 in 20 overs with few alarms although Handscomb had an absorbing battle with fellow Australian McAndrew. Robinson bowled another five-over spell without reward, and it was Fynn Hudson-Prentice who made the breakthrough when Mulder edged the next ball after reaching his fifty low to second slip.
Sussex belatedly introduced offspinner Carson in the 50th over and he struck with his 11th delivery, an arm ball which Louis Kimber - who dropped down the order after struggling in the opener's role - fatally played back to.
Simpson dropped a difficult chance offered by Ben Cox on 1 and he and Handscomb put on 57, including five penalty runs awarded by umpires Paul Baldwin and Neil Pratt when Handscomb was hit by an errant throw from James Coles as he turned his back, having stepped out of his crease.
Handscomb looked untroubled but Hunt, whose first nine overs cost 62 and included eight no balls, was a totally different proposition when he came back on at the Cromwell Road end.
Handscomb made a rare misjudgement playing too far away from his body and giving Alsop an easy catch before Hunt swept away Leicestershire's tail. Extra bounce defeated Mike's defensive prod and Simpson took an outstanding one-handed catch diving to his left to remove Scott Currie. In his next over Hunt had Josh Hull leg before and walked off with figures of 4 for 70.