Nico Rosberg has said that he is not sure Ferrari have the ability to stay ahead of Mercedes in 2017.

A year ago, it was the now-retired German who was en route to the world championship for Mercedes, but now he is merely watching as former teammate Lewis Hamilton pushes to catch title leader Sebastian Vettel.

"Ferrari did a really great job over the winter," Rosberg told Sport Bild. "But from now on it is a development race.

"And I find it hard to believe that Ferrari can keep up. After Silverstone I already thought 'That's it now' and was surprised to see Ferrari fight back in Hungary.

"But Spa will once again be a real Mercedes track and Ferrari is unlikely to have a chance."

However, Rosberg thinks that Vettel is Ferrari's ace card, even though his German countryman is susceptible to the kind of "extreme" emotional outbursts as was seen in Baku.

"We know that he does not control his emotions sometimes," Rosberg said. "Last year we already saw that on the radio in Mexico.

"But this thick skin in the consequences of such an action is also a strength of Sebastian's. In the short term and in the moment it may seem weak, but over a whole season that can actually make the difference.

"His basic character, in spite of everything else, is poised for victory, and that cannot hurt."

Mercedes currently enjoy a 39-point lead over Ferrari ahead of the Belgian Grand Prix later this month.