On-loan Chesterfield winger Donal McDermott capped an impressive display with the only goal as Northampton were beaten 1-0 at Saltergate.
The 19-year-old - on loan from Manchester City - pounced in a first half that was dominated by the Spireites after Northampton had started well.
Only a desperate block by Mark Allott in the six-yard box kept out a drive from Dean Beckwith but once Chesterfield's midfield took control, the visitors were forced back and they picked up three bookings in eight minutes as they struggled to cope with the home side's movement.
McDermott tormented Northampton and he created several openings before he fired an angled shot from 20 yards past Chris Dunn in the 32nd minute.
Northampton were better in the second half and only a great tackle by Derek Niven halted Steve Guinan but Dunn did well to deny Allott a second in the 58th minute.
Ben Marshall almost equalised with a 74th minute shot that rattled a post but Jack Lester went close to a second in stoppage time.