Late goals from substitutes Armand Gnanduillet and Tendayi Darikwa give the Spireites a 2-0 win and all three points at Fratton Park.
In the first few minutes, defender Joe Devera went close with a header. Moments later, Marc Richards had a goal ruled out for off-side, after a finished past John Sullivan.
Patrick Agyemang and David Connolly were linking up well together in the opposition half. Jay O’Shea then thought he had put his side ahead when his effort looked to have gone over the line, but the referee waved away the visitors protests.
After false claims for a penalty from the hosts, Connolly was then awarded a red card for violent conduct, after he struck Sam Hird in the face in reaction to the challenge.
Chesterfield began to control the game right up to half-time, although ten-man Portsmouth managed to get in level at the break.
After the restart, the home side were awarded a free-kick which failed to beat the wall. A man down, but they still began the second period brighter than the visitors.
There was a lack of opportunities in the second period, causing a drop in tempo as both teams began to pass the ball around in hope of building an attack.
With just under 20 minutes left to play, Gnanduillet was brought into the game in place of Nathan Smith. Moments after entering the fray, he managed to break free of the Pompey defence, but Morsy’s pass failed to reach the striker, who was then caught in possession gifting Portsmouth a great goal-scoring chance, but the shot failed to trouble goalkeeper Tommy Lee.
On 87 minutes, the deadlock was finally broken. A lofted Eoin Doyle cross was met by substitute Gnanduillet with Sullivan rooted to the spot, handing the visitors a late lead.
As the home side were trailing, they threw everything they had at the Spireites defence. The visitors then got their second in injury time, with Darikwa firing a low shot across Sullivan which sealed all three points for the away side.
A win at Fratton Park is no mean feat and Spireites boss Paul Cook will be delighted, looking ahead to the game against his former club Accrington Stanley at the Proact Stadium next saturday.