Nico Rosberg has questioned former teammate Lewis Hamilton's claim that he is fitter than ever amid the corona shutdown.
Mercedes' Hamilton claims he is "fitter and healthier" than ever, even though drivers are in the midst of what will be a five month layoff for the pandemic.
Hamilton also said the period of no races is "almost a blessing".
"He is definitely doing everything he can," Rosberg, who beat Hamilton to the 2016 title and had an acrimonious relationship with the six-time champion, told RTL.
"But I'm not sure if he understands how the body degrades within five months, also in terms of precision and driving ability."
Several F1 drivers have been saying the biggest difficulty when returning in July will be the weakened neck muscles, as well as basic racing "rustiness".
"I see two challenges for the drivers," Rosberg said.
"First of all it is the muscles, especially the neck muscles. It is impossible to train for those forces in the gym.
"But what I think is even more complicated is that they are currently unlearning the perfection of driving," he added.