Essex won the toss and elected to bat. They lost Pettini lbw to Chris Woakes in the third over of the day and in the tenth over, Neil Carter took the wickets of Westley (12), Mickleburgh (10).
England international Ravi Bopara faced nine balls before Carter had him lbw to leave Essex on 31-4.
Rikki Clarke, who was fined three penalty points under the ECB discipline rules for breaching the rules about appealing during Warwickshire's Pro40 win at Leicestershire, made it four lbw's when he had Foster given out as Warwicks were heading for the lunch interval clearly on top.
By the lunch interval, Essex were 83-7 with a catch by Chris Woakes to remove Phillips and Boyd Rankin having Masters caught behind by Ambrose.
Walker went for 29 and McCain became a third wicket for Clarke when he trapped lbw before Essex ended the day on 149-9.