I think the most likely time-line in which a Sino-German war could happen would be with a quick German victory or ceasefire in WW1.
After beating the allies, but not destroying them germany would want to get in on the colonization game. basically all that wasn't already gobbled up was Turkey, Persia, China and Japan. I could see a victorious German Empire wanting to gain some colonies while there's colonies to be got.
The Nazis would not be able to push through all of Russia, Siberia, Central Asia, and Western China ever. There probably isn't enough gasoline in the world for a Nazi Panzer Army to drive across Russia... haha.