If Liverpool are to beat Porto and progress to the Champions League semi-finals, they will have to get past a rejuvenated Iker Casillas.
Casillas will arrive at Anfield for Tuesday night's Champions League quarter-final, first leg as one of the form goalkeepers in the competition, after many had wondered if his career at the top level was over following his move to northern Portugal from Real Madrid in July 2015.
However, the veteran was key in helping Porto top their Champions League group before Christmas, making history after becoming the 'keeper with the most penalty saves in Europe's premier club competition when he kept out a spot-kick from Lokomotiv Moscow's Manuel Fernandes.
The Spaniard also played an integral role as Sergio Conceicao's side squeezed past Roma 4-3 on aggregate to set up this intriguing last-eight showdown against Liverpool, with Porto rewarding him with a new contract as a result.
All of which is in stark contrast to the previous time the two teams met in a Champions League round-of-16 tie last March when Casillas was not even selected for the first leg.
The player had found himself dropped by Conceicao just eight Primeira Liga games into last season, with the World Cup-winner out of Porto's line-up for 20 matches, while he did not even make the bench for one game.
"I can't close my eyes on two weeks of training that did not reach my expectations," the Porto boss said in October 2017. "Iker didn't train with the energy that I demand."
In fact, the shot-stopper only featured in the return leg at Anfield because the game was in effect a dead rubber after Jurgen Klopp's side had destroyed Porto 5-0 the previous week at the Dragao.
Once back in their starting line-up, though, there was no looking back for Casillas, who finished the previous campaign as Porto's No 1 again after helping the club win the league for the first time since 2013 following a spell of four successive Benfica titles.
Maybe that is because, despite arriving at Porto as one of the most decorated footballers on the planet, Casillas still knew what it was like to face the axe after the fractious end to his glittering career at the Bernabeu.
It was, of course, Jose Mourinho who very publicly benched Casillas in favour of Diego Lopez, signed from Sevilla in January 2013, initially as cover for Madrid's injured captain, before replacing him as the club's No 1 despite his subsequent return to fitness.
"Just as Casillas can say he would prefer another coach such as (Vicente) Del Bosque or (Manuel) Pellegrini, I can say that I prefer Diego Lopez," the then-Real boss said four months later. "And while I'm the coach of Madrid, Diego Lopez will play."
Casillas also then lost his place in Real's Liga line-up under Mourinho's successor Carlo Ancelotti, playing just two top-flight matches in 2013/14, before returning the following campaign.
But that was his final season in the Spanish capital, with Madrid president Florentino Perez reported to have forced the Real legend out of the club in the summer of 2015, an action even Barcelona captain Xavi claimed left "a bad taste".
For Casillas, the man dubbed San Iker at Real due to his exploits with Los Blancos, this was not the way he had envisaged ending his 25-year relationship with the biggest club in world football.
A quarter of a century dominated by success in Europe's premier club competition, including a trio of wins in 2000, 2002 and 2014 - the last of which he was captain for, while 17 years ago a 20-year-old Casillas came off the bench to replace the injured Cesar Sanchez and help see Real over the line against Bayer Leverkusen in Glasgow.
So could there be one last Champions League hurrah for Casillas, and possibly even a fourth winners' medal, before he hangs up his gloves?
With Porto considered the rank outsiders of the quarter-finalists, you would say the chances of that happening are remote.
But you would not put it past 'Mr Champions League' to turn back the clock and repel Liverpool's fearsome forward line to set up a possible semi-final showdown with old foe Barcelona later this month.
That would be a fitting finale for someone who was first called up to Real's senior squad for a Champions League clash in Rosenborg while still at school aged just 16.
"I was in design class," he recalls. "I left school, went home, changed my clothes, got in a taxi to Barajas, and I met all the stars - everything you thought impossible when you were a kid. [It felt] like winning the lottery.
"I went from being in class with my mate Julio to sitting at the same table as Fernando Morientes, Clarence Seedorf, Fernando Sanz, Predrag Mijatovic, Davor Suker and Raul. It was something magical."
Casillas was an unused substitute in Norway in 1997, although two years later he did become the then-youngest 'keeper to debut in the Champions League, aged 18 years and 177 days, his first of a record 175 appearances in the competition.
Appearance No 176 comes on Tuesday evening when the Castilian custodian will be hoping to repeat the clean sheet he left Anfield with 12 months ago, as opposed to the 4-0 scoreline he suffered on his first visit to Anfield with Real in March 2009.