Jenson Button has singled out Mercedes as the team that has impressed him the most in preseason testing.
Amid a growing view in the F1 paddock that Mercedes has made a major step forward with its W04 to rejoin the battle at the front of the field, Button too has no doubts about the form of his same-engined rivals.
When asked about which other teams look fast right now, Button said: "I can't tell anything from the tests so far. I don't know who is quick and who isn't. But I would say one team that has been consistently quick and consistent in their runs is Mercedes more than anyone else."
When asked about his reaction to Button's comments, Rosberg said on Thursday: "We are pleased in general. It has been a good winter, but I don't want to speak about any performance level, or where I think I am or where others are.
"It is just not possible to know at the moment really. I think the way you can describe our winter is 'pleased' and that is it."
Although teams are in agreement that there is no definitive answer on who is fastest right now, Button said that everyone was trying to build a picture of the competitive order.
"We all look. We would be lying if we didn't look at times," he said. "That is what the timing screens in all the trucks are for. We obviously look but to try and get an understanding is very difficult of where people are."