Worcestershire 287 (Whiteley 76, Gurney 4-97) and 43 for 0 trail need 419 more runs to beat Nottinghamshire 499 for 9 dec and 249 for 4 dec (Libby 100*, Patel 71, Wessels 57)
Worcestershire face a tough challenge on the final day if they are to avoid defeat against Nottinghamshire in their day-night pink ball Specsavers County Championship match at Trent Bridge.
Set to score an improbable 462, from a minimum of 113 overs, they reached the close of the third day unscathed on 43 without loss.
They have been set a challenging target after Jake Libby had fluently progressed to the fifth century of his career. The 25-year old opener reached the landmark from 144 deliveries, with six fours, at which point the hosts declared their second innings closed on 249 for 4.
Samit Patel, who made 76 in the first innings, scored 71 and shared in a second wicket stand of 121 with Libby.
Earlier, Worcestershire's first innings closed on 287 all out, with Ross Whiteley having made 76.
At the start of the day, the home attack needed only 70 minutes to pick up the final three wickets but elected not to enforce the follow-on despite securing a first innings advantage of 212 runs.
Harry Gurney took the first wicket to fall, thereby securing figures of 4 for 97. Chris Nash, with his first two wickets for the county, ended with 2 for 4.
Whiteley took the early plaudits, clubbing two huge sixes in a 42-ball half century. The pugnacious left-hander eventually fell for 74, his highest first-class score for three years.
Dillon Pennington, a 19-year old seamer on debut, dismissed Nash, who made a hundred on the first day and later added the scalp of Billy Root.
During Patel's innings he was reprieved by a great show of sportsmanship from the visiting captain, who threw down the stumps with the batsman stranded short of his ground.
Patel had accidentally collided with bowler Whiteley and had his progress baulked. The umpires began conferring but Brett D'Oliveira quickly intervened and withdrew the appeal.
An apparent rib injury to Ben Cox forced the Worcestershire keeper to hand over the gloves to Joe Clarke and leave the field. Patel hit the spin of Ben Twohig into the hands of cover and then Ross Taylor, playing his final innings for Notts, was bowled by Daryl Mitchell for just one.
Riki Wessels applied the necessary firepower to extend the lead over 450 and was unbeaten on 57 at the time of the declaration.
Mitchell and Martin Guptill safely negotiated the tricky twilight stage at the end of the day and will begin anew in more favourable conditions on the final afternoon.