Jason Roberts' second-half double sandwiched a rare goal from captain Ryan Nelsen as Rovers bounced back from Jamie O'Hara's shock early opener to finally clamber out of the relegation zone.

And Allardyce admitted he is now relishing the prospect of the international break before his side embark on "phase two" of their season with a bid to move all the way up to European contention.

Allardyce said: "I've been fighting ever since I've been here. It was a hugely difficult start for us and we've been battling away all season. We need that extra six or seven days of breathing space.

"Now we're in phase two - we've played the first game of phase two and we've got three points on the board and it's taken us out of the bottom three.

"We can have a good few days getting ready for the November, December, January spell because it could be a mammoth game scenario for us, especially if we get through to the semis of the Carling Cup."

Allardyce joked that he had earned his money after making a radical move at half-time to introduce Roberts and Benni McCarthy in place of his two wide men in order to match Pompey's problematic diamond midfield.

O'Hara's low, long-range effort in the 15th minute had bobbled past Rovers keeper Paul Robinson in the only action of note in a dismal first period which the visitors just about shaded.

Pompey, buoyed by last week's thumping 4-0 win over Wigan, appeared to have got their tactics spot-on and their defence dealt well with Rovers' set-pieces, but the second half was a different story.

Goalscorer O'Hara told pfctv.com: "They brought on Roberts at half-time and that changed the game. They really put us under pressure and were bombing it into the penalty area and we couldn't deal with it."

Rovers, booed off at the break and perhaps fortunate to still have 11 men after Pascal Chimbonda escaped with a booking for blatant retaliation on O'Hara, looked a different side from the second half whistle.

It took just seven minutes for Allardyce's brave change to pay off when Nelsen headed down a David Dunn corner in the box and Roberts reacted quickest at close-range to steer the equaliser home.

Nelsen scored only his second goal in almost five years at the club when he shrugged off defensive attention to meet another Dunn corner with a bullet header past James in the 73rd minute.

And as Pompey threw caution to the wind Rovers wrapped up the points five minutes from time when excellent work from McCarthy led to a poised left-wing cross which Roberts gleefully tucked away.

Allardyce added: "We all knew what a big game it was today because if Portsmouth won we could have gone bottom. It put immense pressure on us, it was our biggest game this season, but in the end we came through with flying colours."

Pompey boss Paul Hart maintained his side have made great improvements since their own poor start to the season and will now have the international break to try to restore some of the fragile confidence his side had gained.

O'Hara added: "We were sitting there at half-time thinking we could be out of the bottom three going into the international break.

"But now we go away rooted to the bottom with 15 days to think about it before our next game. But we've now got to go back into training and pick ourselves up."