Essex 339 (Walter 134, Critchley 53, Potts 4-71) and 28 for 1 need a further 405 to beat Durham 587 and 184 for 8 dec (Borthwick 71, Harmer 4-75, Porter 3-18)
Wicketkeeper Michael Pepper equalled the Essex record for the most Vitality County Championship dismissals in a match as his side were set 433 to win on what looks like being an intriguing final day.
Pepper had taken four catches in the first innings and followed it up with another quartet and a stumping in the second to equal the county's best of nine in the match.
Earlier, Paul Walter had completed the second century of his career but Matthew Potts' exceptional fast bowling gave Durham a 248-run first-innings lead.
The visitors did not ask Essex to follow on, and Jamie Porter extended his lead as Division One's leading wicket-taker with three scalps to take him to 32 wickets this season, with Simon Harmer picking up four before Durham declared eight down.
That set Essex 433 to win in a minimum of 107 overs, and in the 11 overs in the evening, Potts nicked off Nick Browne as the hosts reached 28 for 1 with 405 still required to win.
Walter had been restricted to two Championship appearances this season after hyper-extending a toe on his left foot. The left-hander had saved Essex with Matt Critchley the previous evening but their 147-run stand ended early on day three when Critchley slapped to mid-off having failed to settle in the morning.
Walter continued with a lovely array of drives to reach a 146-ball century, the second of his first-class career and much quicker than his 263-ball effort against Yorkshire two summers ago.
Potts has slowed his run-up down in recent times but has not lost his pace and was electric in the morning. He had only taken 11 wickets in his previous five outings this season, as his hopes of taking James Anderson's spot in the England Test team diminished.
Potts had already accounted for Browne and Jordan Cox before beating Pepper and Walter for pace to end up with a season-best 4 for 71.
That left Essex 310 for 7, and the final three didn't stick around. Shane Snater drove to cover, Harmer edged a ball that kept low behind and Eathan Bosch skewed to short third.
Durham's decision to bat again backfired initially as they slumped to 11 for 2 and then 77 for 4 with Porter continuing his season haul.
Unlike Potts, Porter's England dreams have seemingly faded but his appetite for wickets hasn't lessened. He found a subtle amount of nip away to find Alex Lees' outside edge first ball of the innings, then extracted a feather edge from Michael Jones' bat in the seventh over. He could almost have had a couple more had he had a bit more luck with some chop-ons.
The pitch, which had been a batting paradise, has developed some variable bounce and turn as the innings have progressed - which Harmer and Critchley exploited. Harmer got his wicket by teasing David Bedingham with a flighty delivery outside off stump, with the ball lobbing to Tom Westley at cover.
Critchley also got first-innings centurion Ollie Robinson chasing a wider ball to edge behind, before Porter returned after tea to castle Colin Ackermann.
The steady trickle of wickets was juxtaposed by Scott Borthwick's 88th first-class fifty; a picture of serenity, simply picking off the wayward balls for 71. But his vigil was ended by Harmer, with Pepper grabbing his eighth catch of the match, and then Ben Raine slog-swept into the deep two balls later.
Pepper stumped Bas de Leede to bring about the declaration, to join Kenneth Gibson, David East and James Foster as the only Essex keepers to effect nine dismissals in a match.
Browne edged Potts - who ended the day with 1 for 9 from his five overs - behind as Durham made an early inroad.
