"Maybe they are not showing us everything they are capable of," Germany's DPA news agency quotes him as saying.
As Mercedes' rivals apparently catch up with the Brackley team, however, championship leader Rosberg admitted he smells tactics at play.
"We will certainly not make the mistake of writing off Red Bull," he told Der Spiegel.
"They are trying to convey the impression that they have little chance of the championship," Rosberg explained. "All tactical, I think.
"I am quite sure that Sebastian and Daniel (Ricciardo) will win races this season," he added.
On the other hand, perhaps Mercedes is also playing its own tactical game.
McLaren's Jenson Button said on Friday that the German squad should not be overly concerned about the development pace of its rival teams.
"It is not like Mercedes are going to stand still," said the Briton.
"It is going to be half a season before anyone else can challenge for a win," Button argued.
Former F1 driver David Coulthard agrees, saying he has not seen one team with such a gap over the field since he drove for McLaren in 1998.
"But even then, and also in the Ferrari era, they weren't this far ahead," he told Austria's Laola1. "At that time it was half a second or one second per lap, but Mercedes is almost two seconds faster than anyone else.
"I very much hope that Ferrari and Renault can catch up," said Coulthard, "otherwise it will be a very one-sided year."