Sebastian Vettel is emerging as a favourite to become Nico Rosberg's longer term replacement at Mercedes.

However, the German has said that he is definitely not leaving his Ferrari deal before 2018. That ties in with speculation that a one-year deal for Lewis Hamilton's new teammate next year is likely.

Mercedes's Toto Wolff told Sport Bild: "There are some well-known drivers without a contract for 2018, so I think we have to think about drivers like Vettel."

According to Red Bull's Dr Helmut Marko, Mercedes need to think seriously about 2018 already, because Hamilton might not be committed for the future.

"There are rumours that Hamilton could say 'Now I'm going to be an actor'. What will they do then?" he is quoted as saying by Kolner Express.

Yet what seems clear is that Max Verstappen and Fernando Alonso - two other obvious Mercedes targets - are not heading to the German giant in the foreseeable future.

Spain's Marca sports newspaper quotes a source as saying that Verstappen "is convinced he will be champion in 2017" with Red Bull.

"He wants to win several titles and then cash in", the source added, potentially referring to a later switch to Mercedes or Ferrari.

The report says that Verstappen's current Red Bull deal is for 2017, 2018 and 2019.

Another top driver who could be out of the picture for the next few years is Alonso, with reports that new McLaren chief Zak Brown wants to offer the Spaniard a new three-year deal.

That plan is also backed by Honda, with F1 boss Yusuke Hasegawa quoted by Diario Sport as saying: "We have to give him confidence that he is choosing the right team for the future."

Vettel, therefore, could be Mercedes's obvious top target for 2018.