Worcestershire 186 (Leach 53*, Hutton 6 for 57) and 48 for 4 (Sanderson 4 for 13) trail Northamptonshire 376 (Wakely 102, Pretorius 111) by 148 runs
Moeen Ali resorted to bowling two overs of medium pace as he endured a difficult return to first-class cricket following his omission from England's Ashes squad.
After a torrid Test match at Edgbaston, Moeen took a "short break" from cricket - which lasted all of two Vitality Blast matches - having been left out of England's squad for Lord's.
He bowled 39.1 overs, but only managed three tail-end wickets against Northamptonshire, and surprised many by bowling two overs of seam-up swing bowling with the wicketkeeper standing back shortly before tea.
As Moeen struggled, centuries from Dwaine Pretorius and Alex Wakely and an irresistible new ball spell from Ben Sanderson set Northamptonshire firmly on course for victory.
Pretorius made 111 on his Championship debut and Wakely 102 - his first hundred of the summer - to help Northants take a first-innings lead of 190 before Sanderson claimed 4 for 13 in nine overs to leave Worcestershire 42 for 4 at the close, trailing by 148.
It was a second dominant day for the home side who ground out 123.1 overs with the bat to make 376 before Sanderson seized his chance with the new ball in 17 overs Worcestershire were left at the end of the day.
He drew edges from Daryl Mitchell to second slip for 4 and from Jack Haynes to the wicketkeeper for 19. Another one nipped away to flick the off stump of Callum Ferguson for a four-ball duck before he brought one back to pin Alex Milton lbw for an eight-ball duck.
It was a wonderful spell of nine overs, five maidens, 4 for 13 which left Worcestershire with much to do to avoid an innings defeat.
Northants' day was set up in the morning session by Wakely and Pretorius, who arrived at the wicket for the start of play after Nathan Buck was removed from the game after being struck on the head on the first evening.
Pretorius got off the mark straight driving Wayne Parnell for four and went back to cut Moeen's first ball of the day past extra-cover. He slog-swept Moeen over midwicket for six but then should have been held on 25 when he lifted the offspinner to mid-off but Joe Leach spilled a straightforward chance.
Moeen then went round the wicket and Pretorius sent him over deep midwicket again and drove him wide of point to put Northants into the lead. A short-arm pull past mid-on for four and a flashing drive through cover point brought him a seventh four an fifty in 67 balls.
Resuming after lunch on 70, Pretorius lustily drove Parnell through cover point and next ball flicked him past midwicket for another boundary. Leach bowled short and wide and was cut hard past extra cover to take Pretorius into the 90s.
A flick against Parnell past mid-on for four brought him closer to three figures, which he reached with a push into midwicket in 136 balls with 14 fours and those two slog-swept sixes against Moeen. He swung Ed Barnard to point soon after, becoming Worcestershire's third wicket with the second new ball.
The first of those was Wakely but only after a hard-earned ninth first-class century.
Wakely resigned the captaincy back in May and has enjoyed some reasonable form since but this was his first major contribution to a Championship match.
Returning on 63, he began his work for day two with a crunching back-foot drive for four off Parnell and two clipped threes through midwicket. An on-drive against Parnell took him past his highest score this season before a nudged single wide of mid-off brought him three figures.
It was a grinding effort on a slow wicket in 233 balls with nine fours and a six and Wakely's delight was obvious. But he could only add one to his lunchtime score before shouldering arms to a Parnell inswinger and losing his off stump.
Adam Rossington also lost his off stump for 1 from a beauty from Leach and after losing Pretorius, Northants got stuck, failing to reach a fourth batting point despite only needing 26 in 11 overs. It was the only disappointing element to their day.