After a bad season that saw his Hertha Berlin team relegated, Friedrich, who in the past played as right-back, did not believe he would make it to the June 11-July 11 tournament.
However, strong performances in warm-up games saw him reinforce his position at the heart of the defence.
"Everyone knows I have a tough season behind me. I thought my chances of playing here were only small," the 31-year-old told reporters.
"But I have matured and I feel more comfortable in the central defence. In that position you are more involved in organising the game and that suits me more," he said.
"I am not the coach but I think that the way we played against Bosnia will also be the formation we will play here," he said.
"We support each other and I think Miro is an experienced World Cup player and can best evaluate himself. So I trust him when he says he will be in form. The team is fully behind him," said Mertesacker.
"A lot can happen in three games and so we want to reach a satisfactory number of points very quickly," he said. "So it would be bad if we lose to Australia. That would not be a good start and it is something I do not even think about."
