Speaking ahead of Germany's semi-final meeting with the host nation, Low revealed that Mertersacker had suffered from a cold in the lead-up to the quarter-final clash with France but claimed the central defender had been fit to play.
Low selected Mats Hummels and Jerome Boateng in the heart of defence as his team overcame Didier Deschamps' side 1-0 on Friday, after Mertesacker struggled against Algeria in Germany's round of 16 match.
The 29-year-old Arsenal defender had started all four of Germany's games before the quarter-final and Low insisted Mertesacker could return against Brazil in Belo Horizonte.
"Per Mertesacker was a bit ill after the [last] game but that was not the reason why he did not play from the beginning," Low said.
"We had a few tired players and some suffered a cold. But that was not a negative in the end, so the whole team is fit and everybody is able to play."
Mertesacker was widely criticised for his lack of pace, as Algeria went close to stunning Germany in the round of 16, with the Africans eventually losing 2-1 in extra-time.
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