Ched Evans scored a quick-fire second-half brace as Walsall’s unbeaten league start came to an end at the hands of Chesterfield.
The Welsh striker fired the Spireites ahead on 75 minutes with a thunderous right-footed strike, before doubling their lead four minutes later with a deflected shot.
Jon Whitney made four changes to the Walsall side that were held to a goalless draw at Oldham on Saturday.
Rico Henry and Erhun Oztumer missed out through injury, while Flo Cuvelier dropped to the bench and Isaiah Osbourne was rested.
Matt Preston, George Dobson and Kieron Morris were recalled, with club-record signing Andreas Makris handed his first start.
The Cyprus international looked bright from the off, and after firing wide inside the first 60 seconds, he came within inches of giving Walsall the lead on 11 minutes.
A long ball was flicked on by Simeon Jackson for Makris to chase. The young striker checked inside the sliding Ian Evatt and struck a rising 20-yard shot against the bar.
Moussa was next to try his luck, volleying over after an excellent Jason McCarthy cross had only been partially cleared.
The Saddlers had started promisingly, yet they almost fell behind on 15 minutes after a defensive lapse.
Evans dispossessed Preston inside his own box and accurately cut the ball back for Jay O’Shea to strike first time, but Neil Etheridge reacted smartly, pushing the ball away to his left.
Whitney’s Walsall were looking dangerous every time they went forward, particularly on the counter attack.
Makris fired straight at home keeper Ryan Fulton after good persistence from Joe Edwards, and McCarthy then dragged a left-footed shot just wide from the edge of the area.
Danny Wilson’s Chesterfield responded with Gboly Ariyibi bundling his way past Edwards out on the left before firing high into the side-netting from a tight angle.
Walsall threatened again before the half was out; Morris and Dobson linked up neatly with the former then drilling over from long range.
Half-time: Chesterfield 0-0 Walsall
The home side flew out of the traps in the second half and very nearly took the lead on 50 minutes, Evatt heading inches wide at the back post from Gardner’s deep cross.
An excellent Edwards challenge then denied Evans a shot on goal as the Spireites began to crank up the pressure.
Chesterfield went closer still when Gardner’s angled strike was parried by Etheridge, with Ariyibi, under pressure from James O’Connor, unable to turn home the rebound.
Walsall fashioned a couple of half chances just past the hour mark; first McCarthy headed wide from a deep cross and then Morris fired into the arms of Fulton.
The hosts, though, who were looking the more threatening, with the excellent Evans stinging the palms of Etheridge with a rasping right-footed drive from distance.
And it was that man Evans who broke the Saddlers’ resistance on 75 minutes, when he hammered a blistering right-footed rocket past Etheridge from 20 yards.
Chesterfield had their tails up and four minutes later it was 2-0, Evans with a left-footed shot which took a wicked deflection and looped over the helpless Etheridge.
There was now no way back for Walsall as Chesterfield comfortably held on to go top of League One.
Full-time: Chesterfield 1-0 Walsall
Team: Etheridge, Edwards, O’Connor, McCarthy, Dobson, Jackson, Morris, Kinsella, Preston, Makris, Moussa.
Subs not used: MacGillivray, Cuvelier, Ginnelly, Bakayoko, Roberts, Kouhyar, Cockerill-Mollett.
Attendance: 5406 (478 Walsall supporters).