Riise booked Liverpool's place in the last 16 with a stunning 42nd-minute winner after Steven Gerrard had opened the scoring with a first-half penalty.
The European champions had to survive a late Portsmouth onslaught to secure their place in Monday's draw.
New signing Jan Kromkamp was handed his full Liverpool debut following a transfer window move from Spanish side Villarreal.
The Dutch defender received a crude welcome to English football when he was the victim of a cynical late tackle from Portsmouth's lone striker Vincent Pericard inside the first ten minutes, with Pericard picking up the game's first yellow card.
Pompey captain Dejan Stefanovic handed Liverpool the chance to take the lead ten minutes before half-time when he was adjudged to have controlled Gerrard's corner with an arm.
Gerrard calmly stepped up and slammed the resulting penalty to the right of Pompey's debutant goalkeeper Dean Kiely to make it 1-0.
Liverpool looked to have put the game out of Pompey's reach four minutes before half-time thanks to Riise's stunning strike.
Riise raced onto Fernando Morientes's flicked header and unleashed an unstoppable shot from the left-hand edge of the penalty area which flew past Kiely and in to the opposite corner of the goal.
Portsmouth manager Harry Redknapp replaced defender Gregory Vignal with striker Svetoslav Todorov at half-time and the gamble paid off when the Bulgarian striker won the free-kick from which Pompey carved out their goal.
Todorov was bundled over by Kromkamp on the edge of the area and Gary O'Neil's free-kick was nodded on by Liverpool midfielder Momo Sissoko for Sean Davis to head into the top left-hand corner from six yards out.
Stefanovic came agonisingly close to earning a replay 14 minutes from time when he lifted a 25-yard free-kick just over the crossbar.
Substitute Azar Karadas almost equalised in the dying seconds when he narrowly failed to connect to Matt Taylor's cross two yards out but Liverpool clung on for the win.