Famara Diedhiou put the hosts in front on 20 minutes, before full-back Callum Johnson equalised on the stroke of half-time.
In the second half, the impressive visitors looked favourites to go on an win the tie.
But it wasn’t to be, with Chris Martin firing home the Robins’ winner against the run of play with seven minutes to spare.
![Superb handling and some good saves from the keeper.](https://www.thestar.co.uk/webimg/b25lY21zOjUzODE0MjBhLTI1ZDItNDI1YS1hMDgxLWUzZTQ5OTEyMDk5Njo3YzBkMTNlMi02YTdhLTQ5YjgtYWY2MS0xZjQ5ZjIzYTg2Yzk=.jpg?crop=3:2&width=640)
1. Craig MacGillivray - 8
Superb handling and some good saves from the keeper. Photo: JPIMedia
![Some very good crosses and took first Pompey goal well.](https://www.thestar.co.uk/webimg/b25lY21zOmFhNjFmZTQ1LWE0YTAtNDc4Mi1iYzdlLTZhOTRjNzQ3NjU5ZTpiZmM2YzJmMS04ZjJmLTQzYjgtOGU5ZC1jODQzOWZkOTA5NjU=.jpg?crop=3:2&width=640)
2. Callum Johnson - 8
Some very good crosses and took first Pompey goal well. Photo: JPIMedia
![Looked more assured with Whatmough back alongside him in second half.](https://www.thestar.co.uk/webimg/b25lY21zOmZlZDE1ZmYyLTFiNjMtNDQ1ZC1hODJjLTAzZmM2ZmIxMTU2YTo2ZGNjNmMyNy01NzUzLTQ4YTYtYmI4NS0zODdmOTk1ZDViMjM=.jpg?crop=3:2&width=640)
3. Sean Raggett - 7
Looked more assured with Whatmough back alongside him in second half. Photo: JPIMedia
![(replaced by Jack Whatmough on 46 minutes): Had testing first-half and, considering his booking, looked wise to bring off at interval.](https://www.thestar.co.uk/webimg/b25lY21zOjliODRlNTA2LTFmOWUtNGE4Yi04NjM5LWE2MDUzOWE4ZjkwYTpiMjFmZWMxZS1iMzA0LTQ3MzUtODcxOC1hNWQyYjc3MzZkMmQ=.jpg?crop=3:2&width=640)
4. Rasmus Nicolaisen - 5
(replaced by Jack Whatmough on 46 minutes): Had testing first-half and, considering his booking, looked wise to bring off at interval. Photo: JPIMedia