Cape Town - Although Nico Rosberg has hailed Ferrari's pre-season improvements, he reckons when it comes to the in-season development war it is Mercedes who will have the upper hand.
Ferrari began this year's championship with three wins in six races as Sebastian Vettel surged ahead in the Drivers' Championship.
Mercedes, though, came back fighting as they upgraded their W08.
With three wins in the next five races, compared to just one for Vettel, the Brackley squad not only put distance between themselves and Ferrari in the Constructors' standings but have narrowed the deficit in the Drivers'.
"Ferrari did a really great job over the winter but from now on it is a development race," Rosberg told Sport Bild.
"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."