Germany coach Joachim Loew said there was no greater challenge than defending the World Cup title ahead of the reigning champions' opening game against Mexico on Sunday.
No country since Brazil in 1962 has successfully retained the World Cup, with Italy the only other side to win back-to-back trophies, in 1934 and 1938.
"It's the most difficult feat and history has demonstrated that, no one in 60 years has done it," Loew told reporters at the Luzhniki Stadium on Saturday.
"Teams develop and change, players finish their careers and you must bring in new players which makes it the most difficult achievement."
"Everyone believes in themselves and the path we've come along together. It's always been good for us if we've started the tournament with a victory."
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"At the end of the day it's not really important," he said. "Everyone wants to play well at the start and perform to their best, this is the same irrespective of previous titles."
"He has had two extra training sessions this week, my overall impression is good and that he is fit."