Westley and Ryder revive struggling Essex
Essex 276 and 300 for 3 (Westley 164*, Ryder 90*) lead Northamptonshire 444 (Levi 168) by 132 runs
Ryder contributed an unbeaten 90 to the stand for his second half-century of the match as Essex closed on 300 for three and a lead of 132.
After Northants extended their first innings advantage to 168, despite losing their final four wickets for 40 in the morning, Essex were still 84 runs behind when Westley and Ryder came together shortly after lunch.
Westley was the main aggressor, reaching three figures for the first time in a year off 122 balls, and went on to hit 25 boundaries, while Ryder played the supporting role, following his first innings 87.
Northants had resumed on 397 for 6, but after bringing up maximum batting points with the fourth delivery of the day, Steven Crook fell to the ninth, top-edging a hook off Graham Napier to be caught in the deep for 39.
Richard Levi could only add 11 before he went for 168, caught behind off Napier, and four balls later Rory Kleinveldt was caught at deep point off Reece Topley.
Olly Stone added 20 for the last wicket with Azharullah but became Napier's third victim of the morning as Essex wrapped up the innings inside the first hour with Northants making 444.
Essex lost Jaik Mickleburgh in the third over of their reply without score, trapped lbw by Kleinveldt, but Westley was quickly in his stride reaching 26 as the visitors went to lunch on 48 for one.
The runs continued to flow after lunch until Kleinveldt struck twice in two overs. Nick Browne got a thin edge off the South African to be caught by keeper Ben Duckett for the second time in the game, while Ravi Bopara could only make a single before being bowled by Kleinveldt.
At 84 for 3, Northants might even have held hopes of a three day victory, but Westley and Ryder would go on to dominate the rest of the day on a pitch which proved fruitful for batting with the exception of Essex's first evening collapse.
Westley, who hit 97 against Leicestershire in his comeback from a broken thumb last week, drove Rob Keogh for his 18th boundary to reach three figures in the Championship for the first time in a year and a day. The same over also saw Essex go into the lead shortly before the tea interval.
As in the first innings, Ryder reigned in his natural aggressive tendencies to play the perfect foil for Westley, bringing up his 50 from 115 balls as the 150 stand was reached, the pair sharing Essex's highest fourth-wicket stand in over four years.
Westley passed 150 in 187 balls shortly before the close, while Ryder will resume the final day 10 short of what would be a first century of the season.