"Vettel will have to beware because Leclerc is very strong and he will no longer have the comfort zone of Raikkonen," Rosberg told Autosprint. "Now it will be different, Seb will have th be perfect if he wants to win."
Vettel was right in contention at one stage last year to push Mercedes and Hamilton right down to the wire; but numerous mistakes from Scuderia, as well as individual mistakes from the German, allowed Hamilton and the Silver Arrows to comfortably pull away.
"I do not know exactly what happened to him but this is sport, sometimes it seems everything is going in the right direction, then you just need one mistake to bring you down, you lose confidence and everything goes wrong," Rosberg added.
"The same thing happened to me against Hamilton in 2015. With Lewis, I needed to change my attitude, I decided that I had to be the one to lead te game. Lewis always tends to impose his rules but I took control so as not to allow him to find the motivation. And I won.
"Vettel is superior in terms of dedication to work; he works closely with the engineers. It is a devotion that Hamilton does not have."