Hummels's first-half header was the difference between the European heavyweights, with the Germans advancing through to the last four with a 1-0 victory.
Germany will now face hosts Brazil on Tuesday for a place in next Sunday's final.
"Mats was sensational," Low told FIFA.com. "The way he goes into tackles and wins them and the way he's always in the right place is hugely impressive. On top of that he scored and he's always a threat at set pieces too."
Hummels has scored four goals in 34 appearances for Germany since he made his debut back in 2010.