Hampshire finished the first day in the ascendancy after recovering from 208 for six to make 286 all out and then reduce Essex to 55 for two at stumps.
The visitors lost opener Tom Westley and Ravi Bopara in the first 14 deliveries of their innings and at stumps trailed the joint LV County Championship Division Two leaders by 231 runs.
Essex will reflect on a day of missed opportunity, having put Hampshire into bat in overcast conditions and been in the box seat at the end of the second session. But Hampshire's last four wickets, aided by a significant 41 extras, added 78 runs, to post a competitive total.
Essex faced an hour's batting at the end of the final session and were quickly two for two.
With his third delivery, South Africa Test paceman Kyle Abbott bowled Westley, offering no shot. One run and 11 balls later, Bopara gloved a catch to Michael Bates off the left-arm swing of James Tomlinson to leave Essex in trouble.
James Foster (26 not out) and Nick Browne (19 not out) successfully navigated the remainder of the session to see Essex through to the end without further loss.
It was a day of fluctuating fortunes for Essex as they allowed a promising situation to slip from their grasp. Hampshire, meanwhile, deserved credit for a lower-order fight-back which left them in the stronger position a stumps.
Essex tasted success in the second over of the Hampshire innings, skipper Jimmy Adams being caught behind off the inconsistent Reece Topley to give Foster the first of five victims.
Michael Carberry (35) and Will Smith (34) added 72 before the out-of-favour England left-hand opener was caught behind off the bowling of Tymal Mills, who endured erratic bowling performance, conceding 70 runs off his 12 overs.
Hampshire, 82 for two at lunch, were forced to work hard for their runs - Jesse Ryder bowling an economical spell - and when Smith became Topley's second victim at 107 for three, Foster's decision to put Hampshire into bat appeared vindicated.
James Vince, with a neat 41, and Adam Wheater, who made a pleasing 42 against his former county which included seven boundaries, took the Hampshire total past the 200-mark.
But when Bopara made an arguably overdue appearance into the Essex attack - it took Foster until the 49th over to hand him the ball - he removed both batsmen to reduce Hampshire to 205 for five and, by tea, with Bates also departed, 208 for six..
Essex ought to have pressed home their advantage but instead, Hampshire - aided by a host of extras, which contributed the second highest score - wrested the initiative from the visitors' grasp.
Sean Ervine (38), Michael Coles (27) and Kyle Abbott (24) all made important contributions as Hampshire's lower order added those 78 runs to raise the total to 286.
Bopara finished with a tidy three for 14 off his nine overs, four of which were maidens.
Essex promptly suffered two blows, but rallied to finish the day at 55 for two.