It was certainly not the most fluent performance of the season so far, but the striker’s header proved enough to secure all three points.
Their opponents saw plenty of the ball – particularly after the break – without asking any serious questions of Craig MacGillivray.
Kenny Jackett made one change from the side that visited Bristol City in the FA Cup the previous week.
Jack Whatmough came off the bench in that game, but started against Fleetwood, as Rasmus Nicolaisen dropped to the bench.
Both sides took a while to find their radar, perhaps still a bit rusty having not previously contested a league match in 2021.
But the visitors went ahead with the game’s first proper chance on 16 minutes, as Ryan Williams swung in a fine cross from the right.
It was a difficult delivery to deal with and MARQUIS took advantage, diving forward to head the ball home.
Fleetwood might have grabbed an immediate leveller, but Callum Camps could not beat MacGillivray after Paddy Madden had slipped a neat pass into the box.
Pompey were the side on top, though, and enjoyed a period of possession, without being able to test Joe Hilton between the sticks.
But their opponents gradually started to find their feet and the Blues had to deal with a number of set-pieces sent into the mixer.
They tried to capitalise on a loose ball and Andy Cannon intercepted it and drove forward before finding Williams, whose shot was blocked.
Callum Connolly should have done better at the other end soon after, unable to guide Kyle Vassell’s cross on target.
Pompey were struggling to carve open any more meaningful openings and Marquis could not make firm contact as he tried to glance home a Williams centre.
Cannon and Williams did combine well again before the break, though, and the latter saw another shot blocked by the stretching Camps.
It was the Cod Army who finished the half on top, although Vassell sent two headers off target, while a Josh Morris free-kick was easily dealt with by MacGillivray.
Fleetwood Town 0
The home side were showing more attacking intent immediately following the restart, with plenty of possession in and around the Pompey area.
But it was the Blues who had a chance to extend their lead when Michael Jacobs collected a Marquis pass and fired straight at Hilton.
The keeper then took a poor touch and Williams tried to take advantage, laying the ball off for Marquis, who could not keep his effort down.
It was still an incredibly tight and scrappy encounter, however, with neither side able to find any attacking fluency.
Fleetwood were certainly seeing more of the ball, but some stubborn defending was keeping MacGillivray well protected.
Tom Naylor also flung himself in the path of one fierce strike to ensure the keeper was not called into action.
Pompey were still posing a threat on the counter, though, and Charlie Mulgrew did well to recover after an initial failure to clear had given the ball to Marquis.
The visitors were still holding firm at the other end and when the ball fell to Camps following a spot of pinball, his shot found a path through to MacGillivray.
A Shayden Morris header was also dealt with by the keeper, as the Blues saw out the closing stages to secure all three points.
Fleetwood (4-2-3-1): Hilton; Connolly, Hill, Mulgrew, Rydel; Whelan (Rossiter 72), Coutts (c); Vassell, Camps, J.Morris (S.Morris 79); Madden
Subs not used: Holgate, Duffy, Finley, Matete, Saunders
Pompey (4-4-2): MacGillivray; Johnson, Whatmough, Raggett, Brown; Harness, Naylor (c), Cannon, Jacobs; Marquis, Williams
Goals: Marquis 16
Subs not used: Turnbull, Mnoga, Bolton, Nicolaisen, Close, Hiwula, Harrison
Referee: James Bell