Faced with a 2-0 deficit against AC Milan, Barcelona must achieve the one feat that has eluded it throughout its unprecedented run of titles - the major comeback.
Barcelona's bid for a fourth Champions League title in eight years will be in the balance Tuesday when it hosts Milan in the second leg of the last 16.
The Spanish powerhouse has won 14 titles since 2008, including the Champions League in 2009 and 2011, but fell short in the semifinals against Inter Milan in 2010 and versus Chelsea last season after losing the first leg away.
Barcelona needs to rally at the Camp Nou against a young Milan team that is surging in Italy and looked hungrier and more physically fit than its opponents three weeks ago at the San Siro.
Milan also has history on its side. No team has ever rebounded from a two-goal loss in the first leg without having scored an away goal.
"It's the moment for this team to have a historic comeback,'' said Barcelona defender Dani Alves. "We have had a long week working a lot on getting in top shape, and you can see it because we are recovering our defensive form. What has set us apart from other teams in recent years has been our ability to pressure. We had lost that a bit, but now we are getting it back.''
On Wednesday, 2012 runner-up Bayern Munich hosts Arsenal holding a commanding 3-1 lead from the first leg in London.
FC Porto visits Malaga on Tuesday looking to make good on its 1-0 victory thanks to a goal by Joao Moutinho, who is doubtful for the second game with a muscle strain.
Also Tuesday, German side Schalke hosts Galatasaray with a slim away-goal advantage after their first leg in Turkey ended 1-1.
Barcelona is playing for more than just avoiding its earliest exit from the competition since 2007. It needs a win over a big club to restore its reputation as the leading team in Europe.
Even though Barcelona has the Spanish league well in hand with a double-digit lead, the team is showing its first signs of true vulnerability in five seasons after the loss at Milan and two clear defeats by fierce rival Real Madrid.
Barcelona can take heart that Lionel Messi is on a record scoring run in the Spanish league, having netted in 17 consecutive matches after going on as a substitute in the 2-0 win over Deportivo La Coruna on Saturday and deftly chipping the goalkeeper two minutes from time for his 51st goal this campaign.
That clean sheet against Deportivo was Barcelona's first in 14 matches and boosted confidence that it can repeat the defensive performance against Milan.
Barcelona is also hoping to have playmaker Xavi Hernandez back from a leg injury. Striker David Villa or forward Alexis Sanchez could be new additions to the lineup that lost to Milan after starting on Saturday.
Milan is on an 11-match unbeaten run in Serie A, but will be missing forward Giampaolo Pazzini after he bruised his right leg in a 2-0 win over Genoa on Friday.
Defender Philippe Mexes and midfielder Kevin-Prince Boateng both made the squad that traveled to Spain despite recently struggling with muscle problems.
With Mario Balotelli ineligible after transferring from Manchester City last month, 20-year-old Stephan El Shaarawy, who has scored a team-high 16 goals in Serie A this season, will be expected to lead the attack.
"We've been doing great ever since the holiday break,'' veteran goalkeeper Christian Abbiati said. "This match really comes at a really good time for us. Last season we were able to draw there, then we had two penalties whistled against us in the quarterfinals, so anything can happen.''
Bayern came from behind twice to beat Fortuna Duesseldorf 3-2 on Saturday as it closed in on the Bundesliga title.
If Arsenal doesn't get past Bayern, it will be the eighth straight season in which the team has failed to win a title. Finishing in the Premier League's top four - and therefore qualifying for the Champions League - has been Arsenal's saving grace in its barren years. Bt even that may not happen,with the team five points behind fourth-place Chelsea with 10 games left.
''We have to go there and give ourselves a chance to qualify, with everybody believing,'' said Arsenal manager Arsene Wenger. "We have to try to put Bayern on the back foot. Subconsciously anyway, they think they are through, and we have to subconsciously think that we can do it.''