Shiv Thakor finished the day on 58 not out, after Billy Godleman had earlier hit 49, as Derbyshire held off the Sussex attack on Day Three at Hove.
In the process of reaching a battling half-century, Thakor surpassed 2,000 first-class runs for his career at the age of just 22, having shared a vital sixth-wicket stand with Tom Poynton who hit 29.
Earlier on in the day, Sussex had secured a first-innings lead of 297 runs, declaring on 447-8, before Derbyshire finished the day on 195-6, 102 runs behind.
With the hosts resuming their innings on 342-4, Ben Cotton struck during the first over of the day as overnight centurion Luke Wells was caught behind without adding to his 104.
That brought Ben Brown and Harry Finch to the wicket, both having not faced a ball. Brown played an aggressive innings, but Finch could not follow suit as, on 16, he edged Tony Palladino through to Poynton behind the stumps. Sussex skipper Brown raced to his half-century, before falling on 61 from 54 balls as Matt Critchley castled him to pick up his first wicket of the match.
The young leg-spinner had a second soon after as he had Ajmal Shahzad caught out in his next over for seven, which brought the Sussex declaration on 447-8.
Openers Godleman and Chesney Hughes saw out the remaining seven overs before lunch and then progressed the score to 44 when Hughes was given out lbw to Ollie Robinson for 11.
The visitors lost Hamish Rutherford (3) shortly after, while Wayne Madsen played well for his 23 before he also fell before Tea. Godleman had played a defiant innings for his side, but could not quite make it through to the interval and a third half-century of the season as he edged Robinson behind on 49 with Derbyshire 97-4.
Neil Broom could only add seven, before his wicket brought Thakor and Poynton together with the score having edged past 100. The pair played diligent innings, with Thakor doing the majority of the scoring, and reaching his-half century off 62 balls.
The sixth wicket stand was to end on 79 when Poynton was run out by Wells, before Derbyshire reached 195-6 at stumps, trailing by 102 runs. Thakor was still there at the close, unbeaten on 68, alongside Matt Critchley who was nine not out.
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