Sussex 255 and 236 for 5 (Evans 106*, Brown 64) lead Worcestershire 383 (Cox 61, Ferguson 56, Wessels 55, Leach 54*, Robinson 5.84) by 108 runs
Sussex batsman Laurie Evans emerged as the key figure on day three of an absorbing contest here, yet it was all going so well for Worcestershire and their enthusiastic Chester Road hosts until midway through the afternoon session.
A lead on first-innings of 128 had been established at lunch, the lower order having transformed the scorecard from a modest 226 for 6 late on day two to 383 before Adam Finch became Ollie Robinson's fifth wicket, and Sussex were four down for 34, still well in arrears.
But just as Stiaan Van Zyl was making his way back to the pavilion after Josh Dell had taken a fine catch low to his left at cover, bowler Wayne Parnell was clutching the hamstring in his left leg. The South African has a history of such injuries, including one earlier this season. His immediate departure from the field prompted dismay among home supporters, having seen him bowl well to take four wickets as Sussex slipped to 102 for 6 on the opening day.
It would have been less of a blow had the captain, Joe Leach, another of Worcestershire's key weapons, not himself needed to leave the field not half a dozen overs earlier, nursing a sore gluteus muscle in his upper right leg. Leach bowled again later, but during the period both were out of the attack Sussex were able to lay the foundations for a recovery that saw them end the day 108 in front, five down.
As seems to happen so often, Ben Brown, the Sussex captain, was central in turning Sussex fortunes, anchoring a partnership of 157 in 37 overs with Evans, the former Warwickshire batsman, who lost Brown as a partner when the former top-edged an attempted pull, going on to complete his maiden first-class century for Sussex when he drove Leach past mid-off for his 16th four.
It was a first century in the Championship by Evans since his double-hundred against Sussex for Warwickshire four years ago, since which time he has moved counties hoping to further his red-ball career only to find himself playing largely in the white-ball formats.
It has not served him badly, particularly on the increasingly lucrative T20 circuit, but it has brought frustrations too, since he would like to be seen as the master of all formats. This season, with a vacancy created by the retirement of Luke Wright from the red-ball game, may provide the opportunity.
Speaking with characteristic candour afterwards, Evans admitted he was "overjoyed" to have made his mark at last in a Championship game.
"My first-class career leaves so much to debate," he said. "I don't really know what has happened to it, in a way, and with the waiting I feel like it's great to finally get out there and get a hundred.
"I moved to Sussex for red-ball cricket, primarily, but as it happened I got injured in the first year, I missed a chunk in the middle of the season, and people came in and did well.
"I felt like I might have been due an opportunity last year and I spoke with Dizzy [head coach Jason Gillespie] about it. I felt I had played pretty good cricket all year and my game was in as good a shape as it ever had been.
"I feel ready for the challenges of red-ball cricket. When you get the chance there is pressure on you, having said I felt ready to play. You have to go out and do the business.
"The red-ball game is a lot tougher, it tests your technique so much more, and that's why runs in red-ball cricket feel so rewarding. So I'm overjoyed, really, after all the hard work I've done."
Earlier in the day, the depth of Worcestershire's batting helped them build so substantially on an overnight lead, six wickets down, of seven runs. Robinson again offered the most threat among the Sussex bowlers, giving away few easy runs, but Ben Cox, Brett D'Oliveira, Parnell and Leach all made a significant mark with the bat.
Cox and Leach made half-centuries, the latter finishing unbeaten on 54 after flexing his muscles with eight fours and a six. The last four wickets added 157.
Robinson, who has taken 18 wickets in three matches since recovering from a shoulder injury, made the first breakthrough of the day when Cox pushed forward and edged to wicketkeeper Brown for 61.
D'Oliveira fell leg before to the same bowler before Parnell followed up his four first-innings wickets by stretching the lead in partnership with Leach, the pair adding 68 before the South African was bowled off his legs by Luke Wells. Leach completed his half-century with a pull for six off Robinson.
Leach sent down 10 overs late in the day, appearing to be none the worse for his earlier mishap. How much he can contribute to the final day will depend on whether he suffers any reaction overnight. Parnell's further participation as a bowler, however, looks in doubt.