France captain Maxime Lucu warned on Friday that the heat and humidity in steamy Tokyo will affect decision-making when his side face Japan in the Nations Championship.
Temperatures of around 30C are expected at Tokyo's National Stadium for Saturday's game, with high humidity and possible rain complicating matters further.
France arrived in Japan after thrashing Australia 42-26 in Brisbane last week, and Lucu said the weather has made training "very demanding".
The scrum-half is wary of conditions having an impact in their test against Japan, who lost 36-20 to Ireland last week in Australia.
"Even though Monday was just strength circuits and cycling, we could see the sweat level was through the roof," Lucu said.
"The body is working hard and it affects your clarity when you're out on the field.
"We saw it again in Tuesday's training in the intense heat, with a lot of dropped balls that were wet from sweat and humidity, so we need to be careful about that," he added.
Lucu said there was "a lot of frustration, a lot of guys getting worked up because of mistakes" caused by the conditions in training.
"On Wednesday it was much better, and today even better again," he said.
"It's a personal effort everyone has to make, because right now we'll be lacking clarity due to sweating and fluid loss at a time when we're under pressure."
France started their Nations Championship campaign with a narrow 34-32 defeat to New Zealand in Christchurch, before heading to Australia to beat the Wallabies.
Japan also have one win and one defeat, after beating Italy 27-10 in Tokyo before losing to Ireland.
"It's even more about keeping a cool head at the start of the match," said Lucu.
"We know we'll have plans to counter this Japan team, who will bring a lot of physical battles, a lot of aggression."