The Reds' lacklustre performance hardly mattered, though, after a devastating 5-0 win in Portugal three weeks ago all-but secured a first appearance in the last-eight since 2008-2009.

That first-leg cushion enabled Jurgen Klopp to make five changes to the side that beat Newcastle United 2-0, with leading goalscorer Mohamed Salah dropping to the bench for Adam Lallana, who was making his first start in two months. 

Sadio Mane had two clear chances to open the scoring in the first half, the second of which crashed against the post, while Dejan Lovren headed over.

James Milner and Roberto Firmino also squandered opportunities in the second period and Lovren blocked a late effort from Oliver Torres to ensure the Reds remain one of three unbeaten sides in this year's competition alongside Tottenham and Barcelona.

Liverpool was a long way from its swashbuckling Premier League form and Klopp will hope it has not lost any impetus ahead of the crunch clash against Manchester United at Old Trafford.

After a sluggish opening, the host team burst into life midway through the half with Mane inches away from opening the scoring. The Senegal forward – who scored six goals in his previous three Champions League games – met Joe Gomez's fizzed cross with an acrobatic volley, which flashed agonisingly over. 

Mane went even closer just after half-hour mark when he pulled Milner's pass out of the sky and drove a low half-volley against the inside of Iker Casillas' right-hand post. 

Milner was the architect again soon after, whipping in a teasing free-kick that Lovren could only head just high of the target.

Milner had a chance of his own two minutes after the restart, but he failed to make a clean connection with Lallana's cross and the ball bobbled harmlessly wide.

Porto finally carved out a chance of their own when Majeed Waris shrugged off the attention of two Liverpool defenders to drill a low shot towards goal, which was kept out by the under-worked Loris Karius. 

Firmino saw a goal-bound effort blocked by Porto captain Felipe moments before being withdrawn for Danny Ings.

Salah was brought off the bench with 16 minutes remaining and, unsurprisingly, went close to adding to his 32 goals this season, though Casillas was equal to his drilled effort.

Oliver almost spoiled Liverpool's outing late on, while Ings should have scored his first UEFA Champions League goal with a looping header that Casillas did well to paw away and keep the Reds off the scoreboard.