Worcestershire 368 and 59 for 1 (Libby 17*, Lakmal 1-19) trail Derbyshire 565 for 8 dec (Dal 114*, Masood 113, Guest 77, du Plooy 62, Thomson 54) by 138 runs
The all-rounder made 114 from 177 balls before Derbyshire declared on 565 for 8, their highest ever score against Worcestershire, and a lead of 197.
Worcestershire were left with 16 overs to negotiate and they lost Ed Pollock to Suranga Lakmal before closing on 59 for 1, some 138 runs behind.
They had reeled Derbyshire back in the previous evening so it was vital they kept up the pressure from the start of play but a dropped catch in the third over of the morning handed the home side the initiative.
Luis Reece was on 22 when he skied Josh Baker into the covers where Pollock circled under the ball and then dropped a simple catch.
It was a big moment as Reece and du Plooy proceeded to bat for another nine overs, adding 41 more runs, before the second new ball broke the stand.
Worcestershire had delayed taking it for six overs, a decision which looked even more puzzling when the first delivery from Charlie Morris lifted and moved away to have Reece caught behind.
By the time Ed Barnard trapped du Plooy lbw with a full-length delivery, Derbyshire were only two runs behind and Dal and Thomson turned that into a healthy lead with a seventh-wicket stand of 78 in 26 overs.
Worcestershire’s attack got little out of the pitch although Baker bowled with control in only his 10th first-class game and deserved better figures.
It was an impressive performance from a young bowler who eight days ago was being dispatched to various parts of New Road by Ben Stokes.
Before this game, he had never bowled more than 30 overs in an innings so he can look back on this day with some satisfaction when he celebrates his 19th birthday on Monday.
It was Baker who broke the stand, having Thomson lbw as he played back but Dal and the tail put Derbyshire firmly in the box seat.
Sam Conners stayed with Dal for 15 overs before he was stumped charging Baker in the last over before tea and Derbyshire piled on the runs in the final session.
When Dal pulled Ben Gibbon for four, it was the first time in Derbyshire’s history they had posted two scores of 500 or more in a season at Derby.
Dal cut Gibbon for his ninth four to reach his second first-class century and with Ryan Sidebottom, batting with a runner because of a calf injury, added 72 before the declaration came
Although Derbyshire were a bowler down and the pitch remains a good one for batting, Worcestershire have to overcome scoreboard pressure to save the game
Pollock took three fours from the first over by Conners but was comprehensively bowled when Lakmal cut one back in the sixth over.
Derbyshire were unable to make further inroads and with Sidebottom out of action, will need a big effort from Lakmal and the rest of the attack to force victory on day four.