Watch highlights of day one
Opening batsman Nick Browne hit an unbeaten 106 to put Essex into a commanding position after Jesse Ryder helped to skittle Division Two leaders Worcestershire for 84 on the opening day of their LV= County Championship encounter at the Essex County Ground.
New Zealander Ryder returned career-best figures of 5-24 as the visitors were dismissed cheaply in a dramatic opening session.
His wickets came during the course of an inspiring personal spell of 10 overs before Essex, who are contesting the other promotion place with Hampshire, ended the day on 198 for three - a lead of 114 runs.
After David Masters had Richard Oliver snapped up at second slip with the fourth delivery of the morning, Ryder embarked upon his destructive course in his fourth over as Moeen Ali was caught behind, and he went on to remove Daryl Mitchell, Alexei Kervezee, Tom Kohler-Cadmore and Ben Cox in consecutive set of sixes.
In between, Ryder also held a slip catch to dismiss Tom Fell off Jamie Porter for seven as the visitors crashed to 31 for seven.
Worcestershire were saved from utter humiliation by Joe Leach who played boldly for a quickfire knock of 39, containing five fours, from only 30 deliveries. He was one of only two visiting batsmen to reach double figures.
But he was eventually undone by the return of Masters, who held on to a fiercely driven return catch. Graham Napier polished off the innings as he finished with 2-18 - figures that were identical to those of Masters as the visitors were bowled out on the stroke of lunch.
Essex soon lost a couple of early wickets. Tom Westley was run out by Brett D'Oliveira's direct throw with only five on the board, while Greg Smith became a 50th championship victim of the season for Charles Morris with the total on 26 when he was caught by Mitchell in the slips.
But any hopes Worcestershire had of plunging Essex into further trouble soon disappeared.
As the pitch dried out, Browne and Ravi Bopara batted with increasing assurance to carry their side into the lead without further setbacks.
Browne was the main aggressor, despatching anything loose on his legs with power and confidence as he completed his half-century from 141 balls. Bopara, meanwhile, was content to play a supporting role, a fact emphasised by him needing 50 balls to move into double figures.
Bopara stepped up a gear after tea and had reached 31 out of a partnership of 85 in 37 overs when he got a lifting Moeen delivery and was caught at first slip by Mitchell.
Browne's third century of the summer arrived from 209 deliveries and the 23 year-old was still there at the close with skipper James Foster, who reached 36 not out at the end of a rewarding day for his side.
Essex head coach Paul Grayson said: "Obviously it was a good toss to win but our bowlers were magnificent. Ryder, in particular, put the ball in the right areas to take full advantage of the help that the pitch gave him.
"He has been magnificent for us this season and I must confess he has surprised us all with the ball. We signed him primarily as a batsman but he has developed into a fine all-rounder and, hopefully, he will be back with us next season."
Worcestershire director of cricket Steve Rhodes said: "The toss was crucial and unfortunately we lost it. With the ball swinging all over the place, it was always possible that 10 wickets would fall in a session.
"Batting was much easier as the day wore on and obviously we now face an uphill battle."