"We have shown who we are, we put on a spectacle and played a great game," said Messi after the match.
"But we came out cold and looked nervous in the first five minutes. It was a bit weird until we regained control of the game and we scored the first goal.
"We cannot come out like that in a Champions League match because we have the experience of Rome last season and it could (have) made qualifying difficult. Five bad minutes can knock you out."
Barcelona go through to the semi-finals for the first time in four years, where they await the winners of Liverpool and Porto, who play on Wednesday. Liverpool carry a 2-0 lead from the first leg at Anfield.
"It will be difficult because these are the best teams," Messi said.
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