Lomana LuaLua's injury-time equaliser gave Portsmouth a point at Anfield.
Steven Gerrard's thunderous free-kick after 70 minutes looked to have given Liverpool a vital victory.
But the DR Congo's LuaLua headed home from close range with seconds left after goalkeeper Jerzy Dudek badly misjudged a cross from substitute Matthew Taylor.
Liverpool were left cursing a host of missed chances, with Pompey keeper Jamie Ashdown saving from Harry Kewell, Dietmar Hamann and Antonio Nunez.
Liverpool boss Rafael Benitez made six changes in a bid to freshen up his side after the disappointment of the derby defeat against arch-rivals Everton.
Dudek returned in place of Chris Kirkland in goal, who was ruled out with a back injury - although his place was also under threat after a run of poor form.
But the lacklustre atmosphere inside Anfield was not lifted by a subdued opening 45 minutes.
The inspirational Gerrard almost created an early opening with a cross that was palmed away by Ashdown, then Hamann was off target from close range.
Portsmouth's main threat was LuaLua, who saw a shot deflected wide by Steve Finnan and was then denied by Dudek.
And as Liverpool's frustration grew, Gary O'Neill almost broke the deadlock three minutes before the interval, firing a right-foot shot just wide.
Liverpool, as they had to do, stepped up the pace after half-time and a brilliant turn and shot from Gerrard was just over the top.
And as the pressure mounted, Kewell also came close after a surging run.
Liverpool were much brighter after the break, and almost went ahead with the best move of the game after 56 minutes.
Xabi Alonso's brilliant pass was headed into the path of Nunez by Gerrard, but he shot straight at Ashdown.
And Ashdown rescued Pompey again seconds later, blocking a point-blank header from Hamann as he raced on to Djimi Traore's cross.
Kewell had come alive after the break, and he was twice close to finally piercing the Portsmouth rearguard, firing narrowly over from 20 yards then forcing Ashdown into a save low down to his left.
Liverpool at last made the breakthrough with 20 minutes left, and inevitably Gerrard was the scorer.
Hamann touched a free-kick into his path and his right-foot shot flashed into the top corner past Ashdown.
But as Liverpool held on, Pompey struck with a sucker punch.
Dudek was badly caught out by Taylor's driven cross, and the ball bounced invitingly for LuaLua to head home from inside the six-yard box.
Liverpool: Dudek, Finnan, Hyypia, Carragher, Traore, Nunez,
Hamann, Gerrard, Alonso, Kewell, Baros. Subs: Riise, Josemi,
Sinama Pongolle, Mellor, Harrison.
Portsmouth: Ashdown, Griffin, De Zeeuw, Primus, Stefanovic,
LuaLua, Faye, Stone, O'Neil, Berger, Fuller. Subs: Hislop,
Taylor, Berkovic, Mezague, Cisse.
Referee: M Clattenburg (Co Durham)