Defeat would have left Liverpool with a mountain to climb but Rafael Benitez's side secured a victory last night which keeps them in sight of the Premiership's valuable fourth spot.
Fernando Morientes and Luis Garcia scored first-half goals which pushed Liverpool back above Bolton into fifth place, three points behind Merseyside neighbours Everton.
However, Benitez, who eased Steven Gerrard back towards full fitness by bringing him on as a second-half substitute, knows that his side's hopes of a Champions' League finish depend on other teams' results.
"We must take 12 points from our remaining [four] games," Benitez said. "That is the only way it will be possible for us. We know we cannot worry about what happens in other games. We can only concentrate on our own games.
"However much we are looking forward to the Champions' League semi-final against Chelsea, the Premiership games are vital for us. We know we could get to the final but not qualify for next season, but if we finish in fourth place we will make it into the next Champions' League."
There could have been three goals in the opening three minutes. Within 60 seconds of the kick-off the Portsmouth striker Lomana LuaLua left Liverpool's goalkeeper Jerzy Dudek on the ground before placing the ball in the net, but the assistant referee ruled offside.
Liverpool swiftly counter-attacked but Milan Baros was denied by goalkeeper Jamie Ashdown after intercepting a back pass from the home captain Arjan De Zeeuw.
After three minutes, however, Liverpool did take the lead. John Arne Riise's low cross from the right was met by Baros, whose effort was saved by Ashdown , but Morientes managed to get the ball over the line under pressure from Aliou Cissé.
Alain Perrin was unbeaten in his previous two matches since taking over at Portsmouth, and after Riise and Morientes had wasted good chances to double Liverpool's advantage, the home side were back on level terms.
Moments after LuaLua had seen a lobbed effort clawed from underneath the bar from Dudek, striker Diomansy Kamara reacted quickly to score for Perrin's side. Steve Finnan cleared a De Zeeuw header off the line, but Kamara slid in to force a 34th-minute equaliser.
From the outset Morientes and Baros had looked sharp for Liverpool, but it was Spanish playmaker Garcia who produced a vital goal on the stroke of half-time as he rose to meet Riise's left-sided cross and leave Ashdown with no chance at his near post.
Three points would have given Portsmouth great heart ahead of their derby with Southampton but, apart from a Steve Stone chance which fell behind him and a long-range drive from Patrik Berger, they failed to frighten Liverpool.
"The game was just as difficulty as I expected," Perrin. admitted "However we can still take positives from this game. Liverpool are a good team and still in the Champions' League, but we played well and I am pleased with our players."
Portsmouth (4-4-2): Ashdown; Cissé, Stefanovic, De Zeeuw (Primus, 45), Taylor; Stone (Berger, 64), Skopelitis, Hughes, O'Neil; Kamara, LuaLua (Fuller, 77). Substitutes not used: Hislop (gk), Rodic.
Liverpool: (4-4-2): Dudek; Finnan, Hyypia, Carragher, Traoré; Luis Garcia (Smicer, 45), Alonso, Biscan (Gerrard, 65), Riise; Baros (Cissé, 79), Morientes. Substitutes not used: Carson (gk), Pellegrino.
Referee: H Webb (S Yorkshire).