Leyton Orient took a huge stride towards safety as Shane Tudor's winner stretched their unbeaten run to five and left Chesterfield in trouble.
Tudor pounced in the 53rd minute when he chipped the ball over Barry Roche.
Chesterfield wasted several chances with Jamie Jackson denied by keeper Glyn Garner and later having an effort cleared off the line by John Mackie.
But their best chance fell to sub Paul Shaw whose late header was brilliantly pushed over the bar by Garner.
Chesterfield: Roche, Bailey (Shaw 63), Downes, Hazell, Grimaldi (Larkin 84), Davies, Allott, Niven, Daniels, Allison (Hall 69), Jackson.
Subs Not Used: Picken, Critchell.
Leyton Orient: Garner, Miller, Mackie, Saah, Lockwood, Tudor, Easton, Guttridge, Corden (Tann 87), Alexander, Jarvis (Demetriou 90).
Subs Not Used: Morris, Mulryne, Thelwell.
Booked: Lockwood, Tann.
Goals: Tudor 53.
Ref: C Oliver (Northumberland).