Famara Diedhiou put the hosts in front on 20 minutes, before full-back Callum Johnson equalised on the stroke of half-time.
But it wasn’t to be, with Chris Martin firing home the Robins’ winner against the run of play with seven minutes to spare.
1. Craig MacGillivray - 8
Superb handling and some good saves from the keeper. Photo: JPIMedia
2. Callum Johnson - 8
Some very good crosses and took first Pompey goal well. Photo: JPIMedia
3. Sean Raggett - 7
Looked more assured with Whatmough back alongside him in second half. Photo: JPIMedia
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