Sussex 50 for 4 (Cummins 3-19) trail Worcestershire 306 (Mitchell 75, Cox 69*, Magoffin 5-38) by 256 runs
Sussex seamer Steve Magoffin reached 50 wickets for the fifth successive season before Worcestershire fought back on an entertaining first day at Hove.
Both sides are eyeing runners-up prize money in Division Two of the Specsavers County Championship and when Sussex took four wickets in 22 balls in the afternoon to leave Worcestershire on 178 for 7 they looked to have stolen a march on their rivals.
But skipper Ben Cox led an excellent counter-attack by Worcestershire's lower order which saw 128 runs scored in 24 overs for the last three wickets with Cox last man out for an excellent 69 in a total of 306.
They then made inroads into a frail Sussex line-up which, in the absence of the injured Ed Joyce and Luke Wells, contained just four specialist batsmen, three of whom are uncapped.
Miguel Cummins removed two of them - Fynn Hudson-Prentice and Craig Cachopa - for ducks and then had nightwatchman Ollie Robinson leg before in the final over of the day. Chris Nash was trapped on the crease by Joe Leach as Sussex stumbled to 50 for 4 at stumps to trail by 256.
Magoffin's enduring excellence was one of the highlights as the Australian finished with 5 for 38 in 19 overs to take his tally for the season to 54.
His landmark 50th victim Tom Fell was claimed in only his third over when Fell was leg before for a duck playing across a straight one.
Worcestershire had already lost Brett D'Oliveira to a catch at slip in Chris Jordan's first over but from 11 for 2 skipper Daryl Mitchell and Joe Clarke fought back against Sussex's five-man seam attack in a stand of 97 either side of lunch.
Jordan made the breakthrough after lunch when Clarke flashed fatally outside off stump but Mitchell and George Rhodes added 57 for the fourth wicket before Worcestershire collapsed.
Mitchell was caught behind trying to dab Magoffin through the off side and in the next over Tom Kohler-Cadmore endured his third successive Championship duck when he offered no shot to an arm ball from left-arm spinner Danny Briggs.
Magoffin then struck twice in successive overs as Rhodes gloved a lifter and Leach was taken low down at slip before Cox led the recovery.
With Ed Barnard giving him solid support, they plundered 48 in six overs after tea with Cox striking Briggs for three sixes down the ground. After adding 75 in 15 overs, Barnard was held on the mid-wicket boundary but Cummins muscled 25 off 20 balls including an outrageous flat-batted six back over the head of bowler Jofra Archer.
Magoffin was belatedly brought back into the attack and completed his fifth five-for of the season with the second ball of his spell when he bowled Cummins. Cox was last out for 69 from 87 deliveries, which also included nine fours, when he was well caught by the diving Archer at mid-on two deliveries after David Wiese had taken the second new ball.