The Germans set up a mouthwatering semi-final with favourites Spain yesterday with a four-goal demolition of Argentina.
It came just six days after they had put England to the sword in similar fashion and confirmed just how good Joachim Low's young side really are.
Few predicted such heroics when Germany left for South Africa without skipper Michael Ballack, first-choice keeper Rene Adler and midfielder Christian Trasch.
But Mertesacker has been taken aback by the performances of the youngsters who stepped into the breach. "I am very surprised at how well we are doing," said the Werder Bremen defender.
"We are a very young side but that has been our big secret. These players, like Mesut Ozil and Thomas Muller, have come in and grown so quickly in the space of a month.
"Everybody is fighting for the team. We are all behaving like a team and we are all doing our jobs."
Although Germany's squad may end up being split by moves away from the Bundesliga - Mertesacker has been linked with Arsenal, Ozil is attracting the attention of every big club in Europe and Jerome Boateng is on his way to Manchester City - the bond between them is strong.
And no matter where they eventually finish this World Cup, Germany have already served notice that they will be major contenders for Euro 2012, the tournament that will now either make, or shatter Fabio Capello's reputation.
It is rather playing to a stereotype to confirm the German campaign has been a long time in the planning.
But that is precisely what happened, with Joachim Low confirming his own assessment following EURO 2008 was that his side were in need of a radical overhaul - after finishing runners-up.
"We tried to implement a change after the Euros," said the coach. "We wanted to give younger players an opportunity. We were willing to lose a few games - that was a risk we were happy to take - and we have brought about this change. Now we have introduced all these young players and they are fearless."
The only cloud on Germany's horizon yesterday was the soft booking Muller collected in the first half that rules him out of Wednesday's semi-final in Durban.
"I hope that my team-mates get everything right in the semi-final, then I can come back and score another goal in the final," said the 20-year-old.
"To score twice against England, and then get another against Argentina to help the team go further in the competition is almost insane."