Chesterfield lost a third straight game as Swindon Town condemned them to their worst defeat of the season.
The Robins, themselves winless in four in league and cup, made two changes from the team that twice gave up the lead to draw at home with Colchester United. Williams and Jake Reeves switched to the bench allowing Jon Obika and skipper Nathan Thompson return.
The visitors’ Louis Thompson shot out of the ground from a great position 30 seconds in and Spireites’ Dan Gardner rattled the side netting soon after as both sides showed early intentions to attack.
Jimmy Ryan, playing further up-field than normal, half volleyed just wide from a Daniel Jones cross and Gardner curled an angled effort close to the target in the second phase from a corner.
Gardner fed Eoin Doyle and Nathan Thompson managed a fine block and Wes Foderingham saved from Sam Clucas from his shot as Chesterfield enjoyed the better of the early exchanges though Obika, hittingv wide with the goal at his mercy.
Gardner received from Oscar Gobern, volleyed a centre to Ryan who himself volleyed a foot over, a great move from the hosts who were looking significantly livelier than in their last two games. Foderingham saved from Morsy as Chesterfield recovered from their near disaster, but a real disaster was just around the corner.
Two 28-yard free-kicks were conceded and respective yellow cards shown to Jones and then Ian Evatt - who will be suspended on Saturday for the Yeovil game - Kasim hit the first just wide but the second was on target and beat Lee to give the Robins the lead. Byrne used his body to find some space in the box before unleashing an unstoppable 15 yarder to double the visitor’s advantage.
Clucas headed over from a Jones header as Spireites looked more ragged and the half-time break provided welcome respite after a rapid downturn in form after a promising start.
Luongo hit a 20-yard effort low and just wide of the upright as the rain pelted down to compound the home fans’ misery. Cook made a double switch to try and add some zip e, Daniel Johnson and Gboly Ariyibi coming on, the latter for his first home appearance and immediately Clucas got a header in that Foderingham gathered.
Michael Smith crossed but Obika was unable to contact. Lee saved from Smith and Jones had to block the rebound from Obika who then headed against the bar from three yards.
Johnson found space but his shot was deflected for a corner that led to an appeal for a push on Evatt but little else.
Sub Williams, fresh on, took advantage of the confusion caused by the excellent Byrne’s run through the middle and he found himself unmarked and was able to stroke the ball home unopposed.
Chesterfield face third bottom Yeovil Town at the Proact on Saturday in a game that’s become almost ‘must win’ as they look to regain the winning formula and the flair that earned so many early season plaudits.
Chesterfield (4-2-3-1): Lee 6; Darikwa 6, Evatt 5, Margreitter 6, Jones 7; Gobern 5 (Banks 74), Morsy 7; Gardner 6 (Ariyibi 52, 6), Ryan 5 (Johnson 52, 6), Clucas 7; Doyle 5. Subs (not used): Hunt, Raglan, O’Shea, Humphreys.
Swindon (4-4-2): Foderingham 6, Stephens 6, N.Thompson 6, Turnbull 6, Byrne 9, L.Thompson 8 (Reeves 81), Luongo 7, Kasim 7, Bell 6 (Toffolo 86), Obika 5 (Williams 68, 7), Smith 5. Subs (not used): Gladwin, Barthram, Belford, Rodgers.
Goals: Kasim 30, Byrne 34, Williams 71
Ref: Duncan Scott (Newcastle)
Booked: Jones, Evatt, Morsy (Chesterfield) Stephens (Swindon)