His comments, in Bild am Sonntag newspaper, follow hot on the heels of the Hungarian grand prix, where Hamilton blankly refused to obey the order to let Rosberg past on a different strategy.
"Last Sunday there was no emergency situation," Zetsche said. "The two were not really bumper to bumper, which is why I can well understand Lewis not slowing down to let Nico past."
He said it is only in 'emergency situations' that the pitwall will intervene with the free battle between the silver-clad pair.
"At Mercedes there are no team orders," Zetsche insisted. "I stand by that.
"The skill of the drivers and a little bit of luck will in the end decide who has the edge.
"It is true however that Rosberg and Hamilton are not to impede each other with different strategies -- for example two stops versus three stops," he added.