Moeen Ali hit his second century of the match but Worcestershire still find themselves battling to save the game against Lancashire at New Road.
Left-hander Mooen was unbeaten at stumps on 101 out of a total of 166-5, meaning the hosts have a narrow 12-run lead after bowling Division Two leaders Lancashire out for 441.
The England Lions batsman became the first Worcestershire player to score two hundreds in an LV= County Championship match for for three years.
He found himself in early after opener had been Matt Pardoe caught behind off Kyle Hogg, while Glen Chapple then had opposite number Daryl Mitchell lbw to leave the score at 15-2.
Thilan Samaraweera (17), Tom Fell (11) and Ross Whiteley (13) offered some support to Mooen, who raced through to 50 thanks to taking 18 off one over from spinner Arron Lilley.
He reached three figures shortly before the close of play, with Worcestershire's hopes of rescuing a draw now resting firmly on his shoulders. Jack Shantry was alongside him on 10 not out.
Lancashire had earlier opened up a 154-run lead thanks mainly to a stand worth 136 between Luke Proctor, who hit his fourth successive half-century, and Tom Smith.
The stand came after they lost overnight batsman Simon Katich (105) immediately when he steered Shantry's first delivery of the day straight to Samaraweera at gully.
The in-form Proctor made 76 before he was the first of four wickets to fall in the space of 45 minutes once the new ball had been taken after lunch.
Smith was the last to go, becoming wicketkeeper Ben Cox's fourth catch of the innings when edging Graeme Cessford to fall for 84. Both Cessford and fellow seamer Alan Richardson finished with four wickets apiece.