Brett D'Oliveira completed his century against Durham at Chester-le-Street before Worcestershire lost three wickets for six runs to off-spinner Ryan Pringle.
In gloomy conditions, with the floodlights again on from the 2pm start, Ben Cox and Ross Whiteley stopped the rot, taking the visitors to 323 for seven at the break, leading by 126.
Resuming on 213 for four, with D'Oliveira on 91, runs came briskly as Moeen Ali played in carefree style.
When Durham posted a short extra cover he chipped Chris Rushworth over the fielder, but was fortunate as two inside edges took him to 50 off 59 balls.
Pringle then got his fingertips to a leading edge off Rushworth at mid-off as Moeen continued to lead a charmed life.
D'Oliveira drove the same bowler for three through extra cover to complete his 159-ball century with 15 fours and a six then top-edged Ben Stokes's second ball of the day over the wicketkeeper for another boundary.
When Pringle was introduced D'Oliveira hit him for four to mid-wicket then lifted a no-ball over long-on for six – counting eight – before whacking a full toss straight to cover to depart for 122.
Following the stand of 130, Moeen swiftly followed, missing a sweep in Pringle's next over to fall lbw for 59.
George Rhodes made only three before sitting back to cut and edging to wicketkeeper Stuart Poynter.
Whiteley was on one when he drove Pringle for a straight six and after more punishment Durham tried the leg-spin of Cameron Steel, whose first ball almost bowled Whiteley on 20. The left-hander was on 27 at the break with Cox on 19.