The five-time winners have a 5-2 advantage in a clash overshadowed by security concerns which marred last week's first leg game in Anfield, and are favourites to reach their eighth final and first since 2007.
Liverpool supporters have been pouring into the city with an expected 5,000 for the 1845GMT kick-off in the Stadio Olimpico with manager Jurgen Klopp promising a display of attacking football.
"You want to play when it is big stakes for the big things, and that is why we are here. It is a great opportunity," said German coach Klopp.
"If you really want to win, you have to accept that you could lose and be brave, not wait for something to happen.
"We need to be active and use their situation much more than they can use it, because they have to win against us and take a risk.
"We are here to fight for our dreams, and we want to go to the final," the Liverpool boss continued.
The winner will face two-time holders Real Madrid in the May 26 final in Kiev.
Liverpool have been training before the game at the ground of Roma's bitter city rivals Lazio with Reds fans gathered in the city centre chanting "we love you Lazio" ahead of the game, but there were no early signs of trouble.
Fan safety in the Italian capital has overshadowed the build-up, with 3,000 police officers deployed after a Liverpool supporter was attacked by hard-core Roma ultras outside the club's Anfield stadium last week and left in a coma.
Dutch midfielder Georginio Wijnaldum said that the Liverpool squad had been "devastated" to hear about the condition of the fan, Sean Cox. "It's a game and everybody has to be safe and enjoy it," he said.
Roma's players showed their support for Cox by wearing shirts bearing the words "Forza Sean" (Come on Sean) during a training session on Tuesday.
Liverpool issued strict instrutcions for fans to restrict their socialising to the Piazza del Popolo and Piazza di Spagna before making their way to shuttle buses which would be given a full police escort after the game.
"Under no circumstances should supporters attempt to walk to the Stadio Olimpico," the club warned, advising the fans to arrive early in the stadium with gates opening at 1500GMT nearly four hours before kick-off.
Klopp had said he "really hoped everyone can walk to the stadium and look forward to an intense, fantastic and very important football game".
The club have also told fans to respect public monuments in the capital, to avoid street drinking which is prohibited, and stay clear of the Ponte Milvio and Ponte Duca d'Aoste, two bridges near the stadium.
On the pitch, Eusebio Di Francesco's Roman side are hoping for another magical comeback after overturning a 4-1 first-leg deficit against Barcelona to put them through to the final four.
Roma -- who last reached the final in 1984 when they lost on penalties to Liverpool -- have not conceded a goal at home in the Champions League this season.
They will be counting on star forward Edin Dzeko to counter Liverpool ace Mohamed Salah.
Bosnian Dzeko has scored in each of his last four Champions League matches -- a record for a Roma player -- to bring his tally to seven in Europe this campaign.
Liverpool -- inspired by former Roma player Salah -- have scored 20 goals in six European away games.
"The desire to make another miracle happen must push us all to give that extra something," said Di Francesco, adding his side "want to try and do something big in a stadium with 70,000 people".