The world champion was at risk of being on the receiving end of the biggest upset in the tournament's storied history, when debutant Cape Verde twice recovered from a goal down to draw level, once in regulation time and once in extra-time.

Messi's 20th FIFA World Cup goal put the Albiceleste in front, but Deroy Duarte's angled drive ensured Cape Verde forced an additional 30 minutes.

Lisandro Martinez restored Argentina's advantage, only for Sidny Cabral to score what was surely the best goal of the tournament to date, curling into the top-right corner to provoke wild scenes in the 103rd minute.

An unfortunate Diney Borges own goal ultimately ended Cape Verde's heroic campaign, but Argentina is under no illusions about the size of the scare it faced.

"The match was very tough, you always have to take the positive, and that is that this team never gives up," Scaloni said.

"I congratulate the opponents and, when I say that there are no easy opponents, it is really like that. We rescued ourselves; the blows did not slow us down.

"They gave everything, this is Argentina, we do understand that. It would have been crazy to lose, but this is how it is. What are you going to do?

"Things can be improved, we'll talk about that. In Qatar, the team received a hard blow [losing their opening game to Saudi Arabia] and kept trying.

"The fatigue of the players was due to the extra minutes and the heat. When you play with your heart, as they play, it helps to cover the lack of oxygen, and today we gave another show of character.

"I think we're fine. Argentina's thing is to know how to suffer and not give up. We must rest, though now we will have less rest, when we should have had more."

Asked if he ever doubted that Argentina would advance, Scaloni said: "I am always cautious. My experience helps me to be like that.

"The game was ugly and could have gone the other way, although we were the ones who carried the weight of the match."

Messi appeared reluctant to celebrate Argentina's triumph after the full-time whistle, with many Argentina players instead consoling their Cape Verde counterparts.

"Today we knew that it was going to be a very tough match, that this team had not lost against Spain, or against Uruguay," Messi said.

"This is what this [FIFA] World Cup, in particular, has been about. It is all very even, and all the matches are going to be very difficult."