Holders Barcelona are in danger of being knocked out of a competition for the first time since April 2008 when they travel to Sevilla for Wednesday's King's Cup last 16 second leg.
Pep Guardiola's side, who won an unprecedented six titles in 2009, must reverse a 2-1 deficit from last week's first leg and failure would see them eliminated from a competition for the first since Manchester United knocked them out of the Champions League at the semi-final stage at the end of April 2008.
Guardiola and the players were predictably upbeat about their chances of getting past the Andalusians after Barca's 5-0 thrashing of promoted Tenerife in La Liga on Sunday, their first win in three matches this year.
"We are full of confidence for the game," Argentina striker Lionel Messi, who scored a hat-trick to take him joint top of the La Liga scoring chart with 12, told reporters. "The three goals are important but the win more so considering how we had begun the year."
However, Barca's shaky start to the game, when they had goalkeeper Victor Valdes to thank for saving them three times, has given Guardiola food for thought.
"Until we got the first goal there was only one team on the pitch - them," he told a news conference. "Coming out and playing like that won't win us anything. I have noticed that some things aren't going well and they must be fixed."
Sevilla, the 2007 Cup winners, are on a poor run of league form and suffered a third straight defeat on Saturday at home to mid-table Racing Santander.
Goalkeeper and captain Andres Palop is hoping Barca's open, attacking style of play will give his side a better chance of expressing themselves than against defensive teams who look to cause damage on the counter attack.
"Barca is a team that wants the ball and creates space and there is a lot of fluidity, as opposed to teams that close play down and have a more destructive style," he said at a news conference on Sunday.
In other ties, Atletico Madrid bid to overturn a 3-0 first-leg humbling when they host second-division Recreativo Huelva on Thursday.