Lucy Quinn scored the only goal of the game for Pompey in the ninth minute and manager Karen Hills was left to reflect on a disappointing first half show that ultimately cost her team.
"We started so slowly and that's happened in all three of our matches against the top three teams, so we have to put that right," said Karen.
"In fact, the first half was probably our worst performance this season.
"We were much better in the second half and pushed for an equaliser. Andra Marcoci came closest when she was denied one-on-one and we had another couple of chances. But we have to learn that we have to play for the full 90 minutes."
The team travel to Lewes on Sunday (2pm). "We'll work hard this week and hopefully we'll rediscover a bit of form, especially in front of goal," added Karen. "We're creating the chances, we just need to put them away. That's been the story of late."
The team's scheduled trip to Charlton on March 12 has been postponed.