Didier Deschamps is concerned his France players will not be able to cope with the sweltering heat of Brasilia in Monday's second round match against Nigeria.
In a World Cup dominated by South American teams, France have been one of the few European success stories so far.
Deschamps' men topped Group E with two wins and a draw, scoring eight goals in the process.
The French were helped by the fact that all their group matches took place either at 1600 or 1700 local time.
But that will not be the case on Monday when they take on Nigeria in the Brazilian capital, where the temperature is expected to hit 30 degrees Celsius.
Deschamps has changed his squad's eating patterns - they had a lunch of pasta and fish at 9:30 on Sunday morning - to try to help them be ready for the early kick-off, but he is still a concerned man ahead of the match.
"Yes, the heat worries me," the France coach told a press conference.
"Nigerians are naturally more adapted to heat.
"The match is at a different time so over the last three days we have been trying to adapt to this schedule by changing our food.
"You need to manage these problems. Yesterday we saw the Brazilians and Chileans after their (early) match finished and they were very tired."
Four years ago France's World Cup campaign descended into farce when the players refused to train the day after Nicolas Anelka was kicked out of the squad over a row with former coach Raymond Domenech.
Deschamps, who took over from Domenech's successor Laurent Blanc two years ago, is happy that his team have made it through to the last 16 without any commotion.
"I'm proud of what the players have achieved," he said. "We need to appreciate this moment.
"This is a very difficult World Cup. Some big nations have already been eliminated, but we're still here."
Mamadou Sakho, who suffered a hamstring strain in the 0-0 draw against Ecuador four days ago, is France's only fitness concern ahead of the match.
"He only ran in training on Saturday," Deschamps said of the Liverpool defender.
"I'll see on Sunday night if he is available."
Nigeria scraped through to the knockout stages of the competition for the first time since 1998 on Wednesday despite the efforts of Lionel Messi, who condemned the Super Eagles to defeat with two well-taken goals.
Like Messi, France forward Karim Benzema has been on hot form at this tournament, scoring three goals and providing two assists.
But Nigeria captain Joseph Yobo, who won his 100th cap against Argentina, sees no reason to be scared of the Real Madrid front man.
"We've just played against the best forward in the world, so we're ready to measure ourselves against anybody," the former Everton and Norwich defender said.
"I don't see what we should be afraid of. I believe in this team, and I think we can do it."