Not even Bayern coach Jurgen Klinsmann expects his side to be able to come back from such a deficit at the Allianz Arena but Argentina forward Messi, who scored twice in the first match, is still urging caution.

"We cannot go into the game believing it is already over," he said.

"They are a good team and have good strikers who are strong in the air. The only thing we can do is play as we always do."

Barca coach Pep Guardiola is set to resist the temptation to field a weakened side for the game, insisting he cannot take any risks in Germany.

"We have an advantage of four goals, but we have to maintain concentration as we still have to play this game," he said.

"The fact that the first leg went so well does not mean that the second game will go so well.

"We still have got to play this game and we have a great advantage, but that does not give us any right to believe that tomorrow will be easy.

Klinsmann has promised to attack from the off and give the fans inside the Allianz Arena something to cheer about and Guardiola is ready to fight fire with fire.

"They will try everything and that is precisely what I would do in their position," he explained.

"We must work hard and have the right attitude and we will go out there to win."

"If we can manage to score a goal, then we have a leg in the semi-final, but I have a lot of respect for Bayern so I think that a lot is still possible."

"Anything can happen in sport."

Barring an unprecedented result - no club has ever overturned a four-goal deficit in the Champions League - it will be Barcelona going through to face either Chelsea or Liverpool in the semi-finals of the Champions League, but Messi is not considering their potential opponents until the job is done properly.

"We have got to win here before the English teams come," he said.

"We know they are hard to play, that they play good football and that they know this competition well, but we cannot think of that yet. We have first got to get through."

Thierry Henry may miss out due to a fever which prevented him from training today. The former Arsenal striker will face a fitness test tomorrow morning, but Guardiola otherwise has a fully-fit squad to choose from.