Following a goalless draw with little between the sides, the second-round fixture headed into a penalty shoot-out at a packed Bypass Ground.
However, Ciaran Martin and Chinedu Agu both missed their spot-kicks, with Aaron Scott grabbing the winner for the hosts, who claimed a 7-6 victory.
And here are our Pompey player ratings...
1. Conal McDermott - 8
Pompey's skipper came out of the box well to deal with danger throughout, while also commanded his area well. Terrific late save from a deflected shot to keep it goalless. Photo: Jason Brown
2. Michael Ani - 7
Produced one superb cross from the right on the stroke of half-time which Sizer headed against the bar. Athletic and physically dominant, was always willing to overlap from right back. Photo: Jason Brown
3. Ashton Sizer - 8 - MOM
The 16-year-old was a powerful figure in the centre of Pompey's defence, superbly coping with the physicality of Romsey's strikers. At the other end of the pitch, his header on the stroke of half-time from Ani's cross struck the bar. Definitely one to watch. Photo: Jason Brown
4. George Wilson - 7
Another first-year scholar, Wilson combined excellently with centre-half partner Sizer and coped well with the physical challenge. Composed on the ball and had an air of calmness about him. Photo: Jason Brown