Neuer's fitness was a concern prior to the tournament, but his performances in Brazil have led to Kahn labelling him the world's best.
"He has proven at this World Cup that he is currently the world's best goalkeeper," Kahn told FIFA.com.
"Everyone knows that he is a wonderful footballer who can play really well. But what is crucial is that in important situations he can rescue the victory. That is ultimately what he will be measured by."
Germany will face the hosts Brazil in Belo Horizonte's Estadio Mineirao on Tuesday in the semi-final.