Sussex paceman Henry Crocombe claimed his second hat-trick in eight days during an evening of tumbling wickets at Chelmsford. The 24-year-old seamer, who is set to leave Sussex for Surrey at the end of the season, finished with figures of 5-39.
Crocombe's performance contributed to Essex's collapse from 171-3 to 177 all out in 26 balls, with his five wickets coming in 14 deliveries without conceding a run. This gave Sussex an unexpected first-innings lead of 98.
Essex had been progressing after tea on the second day of the County Championship match before Charlie Allison became Crocombe's first victim after scoring 56. The collapse has revived fears of a battle against relegation for Essex in Division One.
Sussex extended their advantage to 188 runs by the close of play, despite losing four wickets in their second innings.