Liverpool twice had to come from behind after Portsmouth took the lead through David Nugent and then Hermann Hreidersson, but a half-fit Torres earned all three points for Rafael Benitez.
In a lively opening ten minutes both sides had clear-cut chances with first Nugent setting up Peter Crouch, but the Portsmouth striker didn't read the quality of the pass and his finish was placed straight at Pepe Reina.
Liverpool, without Steven Gerrard and with Xabi Alonso, Dirk Kuyt and Torres all benched, adopted an unfamiliar wing-back system but the extra width brought some joy in the opening exchanges.
Alvaro Arbeloa was afforded far too much time on the right touchline and when he rolled the ball into the path of Yossi Benayoun, the Israeli looked certain to score only to send his finish into the side-netting at David James' near post.
Fabio Aurelio then sent a devilish free-kick past a motionless James only for it to clip the outside of the post and an opening goal looked on the horizon.
However, the pace dulled and both sides resorted to tedious midfield interplay without any genuine end product.
Such was Javier Mascherano's frustration on 26 minutes that the Argentine delivered an audacious strike from full on 35 yards for James to just get a fingertip to.
The second half saw Portsmouth manager introduce a fit again Niko Kranjcar as the home side clearly fancied their chances against a below-par Liverpool.
Ryan Babel though continued the weekend theme of horrendous misses when he completely mis-kicked from two yards out with an open goal from Benayoun's cut back.
That misjudgement looked even more costly as Pompey took the lead when Crouch controlled a long ball up field and sent Nugent free for the striker to bury his finish under Reina.
The Reds soon drew level when a Crouch back pass was handled by James and from an indirect free-kick almost directly on the penalty spot, substitute Alonso rolled the ball for Aurelio to crash a finish into the bottom corner.
Finally Torres was introduced on 74 minutes as Benitez began to desperately seek the win, but they were pegged back further when Nadir Belhadj's free-kick found Hreidersson unmarked in the area.
Kuyt, another second-half sub, drew worried Liverpool level once more when Sylvain Distin failed to cut out a long ball and Torres was able to find space to pick out the Dutchman.
The stage was left then to Torres who, in the dying moments, beat Sol Campbell at the near post to meet Banayoun's cut back and reignite Liverpool's title challenge.