On day three of their Vitality County Championship clash, Sussex declared on 694 for 9 after new captain John Simpson struck a maiden double-century, resulting in the third-highest total in their first-class history.
In their second innings, Leicestershire closed on 86 for 1, trailing by 270.
Simpson, a former Middlesex wicketkeeper, finished on 205 not out. He shared a seventh-wicket partnership of 255 with Danny Lamb, another new face at the South Coast club, whose 134 was also a career-best.
Ari Karvelas came in at No. 10 and scored 55 off 40 balls. Sussex declared with a first-innings lead of 356 in reply to Leicestershire's 338 all out, with 24 overs left in the day.
With the insurance of a second innings, Simpson hoped his bowlers could dismiss Leicestershire again. However, the weather forecast for the final day suggests there may be interruptions to play.
Leicestershire's bowlers contained Sussex effectively with the old ball in Saturday's final session before taking two important wickets with the new one, leaving Sussex six down and still 56 runs behind.
Another wicket early on the third morning would have exposed the Sussex tail, but it did not come. There was only one sustained appeal for a gloved caught behind by Ben Mike against Lamb, which was firmly rejected. There was barely a hint of a breakthrough until five overs after lunch when Lamb, on 85, got a bat to a leg-side delivery from Matt Salisbury. Ben Cox, diving to his left, could only help it to the boundary.
By that time, the seventh-wicket pair had added 150 to Sussex's total. 133 of those runs were scored during a one-sided morning session. The Kookaburra ball is clearly not conducive to bowling in English April conditions, and it sat up to be hit far too often.
During the session, Lamb hit four boundaries over the rope. On the third occasion, he slog-swept the spinner Liam Trevaskis over the seats on the popular side, and the ball was temporarily lost. Leicestershire received a replacement ball, but it was no more responsive.
Simpson reached his fifty from 108 balls and his hundred from 197, passing the bigger milestone when he swivel-pulled Salisbury for his 15th four. This was his 11th century in first-class cricket. The shot brought up Sussex's score to 450.
Lamb, who transferred from Lancashire during the winter, reached his century much faster than his partner, taking only 108 balls and hitting 10 fours and four sixes. Based on the number of people on the Sussex balcony who applauded as he ran for a couple of runs off Trevaskis before raising his bat, he is already a popular figure in his new team's dressing room.
Lamb expressed his gratitude by treating them to some high entertainment. He achieved this by hitting six number five off Mike, launching Trevaskis high over long-on to chalk up the 500 with his sixth maximum (equalling his old career-best of 125), and reverse-sweeping Louis Kimber, Leicestershire's part-time offspinner, for his seventh.
When his luck finally ran out - he was bowled attempting to heave Trevaskis over midwicket - he walked back to the pavilion grinning from ear to ear. For your information, Lamb was the first wicket taken by the former Durham left-arm spinner while playing for Leicestershire. Figures of 1 for 192 may slightly diminish its significance.
Leicestershire's ordeal continued as Jack Carson scored two sixes in a 23-ball 38, and Simpson hit five sixes and 23 fours, reaching his second hundred off 97 balls. The innings ended when Karvelas was caught at long-off from Kimber's bowling.
Leicestershire negotiated the first 14 overs of the second innings without any issues. Rishi Patel was dropped on 19. The introduction of Carson brought a breakthrough. The offspinner's first ball induced an inside edge onto the pad, and Australian opener Marcus Harris was caught at short leg.