Austria has raised the alarm bell for Mercedes, F1's utterly dominant force in 2014.
But although kicking off the season with a peerless double hat-trick of victories, the party came to a stop in Canada when cooling issues struck both W05s.
Team boss Toto Wolff said on Saturday that the problem has not been entirely resolved. And that was before the qualifying session at the Red Bull Ring.
"We are still in front," championship leader Rosberg, although just third for the race start on Sunday, insists.
"Our advantage is not as big as usual this time, but I think this is a track that suits the Williams well and I also believe they are using a qualifying setup," he told Speed Week.
"I am convinced that we are still the fastest car on the track."
But Mercedes has other issues. Many believe that, after the Monaco 'tantrum' and his retirement in Canada, the pressure is starting to show on Lewis Hamilton.
At the Red Bull Ring, he will start just ninth after spinning on his decisive run in 'Q3'.
"Mistakes happen," team chairman Niki Lauda was quoted on Saturday by APA news agency. "He will not sleep so well tonight.
"Hopefully he survives the first lap tomorrow," the triple world champion told Bild newspaper.
Team boss Toto Wolff, however, has even greater concerns than that. He is worried that Mercedes' entire approach to 2014, in light of the utter dominance of the W05 car, needs a major rethink.
"We are still world championship leader and our car is still fast," said the Austrian.
"But we have learned a lesson. It is not only teammates racing. There are also others who are strong."
Wolff's comments are a clear reminder to Rosberg and Hamilton that, perhaps because they are so focused on one another, they are now behind several rivals for the race in Austria.
But Mercedes is also taking a close look at the way it is approaching races, Wolff admitted.
"It's a very competitive environment with a lot of pressure," he said. "We need to look at our whole system."