Tom Westley's previous highest first-class score was 93 not out
Tom Westley hit his maiden first-class hundred to keep Essex's slim hopes of promotion from Division Two alive on the third day at Derby.
Beginning 308 behind Derbyshire on 166-2, Essex moved to 356-9 declared, Westley top scoring with 132.
The home side then reached 100-3, a lead of 218, when the light drew in.
Northants' win over Leicestershire has ended Derbyshire's promotion hopes, but Essex still need to try and force a win on the final day if they are to go up.
And it seems that Essex's fate now depends on what victory target their hosts are prepared to set.
Beginning the day with the follow-on target of 325 as their first objective, the visitors seemed to be making comfortable progress when Westley and Matt Walker (75) extended their third-wicket partnership to 224.
Westley, whose previous highest score (93no) also came at Derby last season, reached his century by pulling Graham Wagg for his 15th four.
But, when he and Walker fell, to Jonathan Clare and Steffan Jones respectively, Mark Pettini, James Foster and Graham Napier followed as Essex slumped from 238-2 to 299-6.
But Ryan ten Doeschate (41) and Chris Wright (24no) settled the nerves by taking the score to 346 before the Dutch all-rounder was run out for 41 going for a second to deep cover.
Danish Kaneria then thrashed Wagg's left arm spin for six over long on to seal the third batting point which signalled the declaration.
Derbyshire then lost opener Wayne Madsen, caught by walker off Wright for a duck, but Chris Rogers (42) was in commanding form to crush Essex's hopes of a home collapse.
Kaneria accounted for Rogers and Greg Smith before the close. But, with bad light drawing in nine overs before the scheduled close, Essex's promotion ambitions could now rely on the generosity of their hosts.
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