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Cape Town - Christian Horner reckons Sebastian Vettel could be on his way to Mercedes in 2018 unless Ferrari up their game.
The four-time World Champion swapped Red Bull Racing for Ferrari in 2015, winning three grands prix in his first season with the Italian outfit.
This year, though, he has yet to get off the mark.
Added to that, Ferrari have fallen behind Red Bull as Mercedes' main rival, leading Horner to question how long Vettel will remain at Maranello.
"I have a feeling Seb looks stressed out," his former team boss told Bild.
"I know from personal experience that he has to feel good in order to perform well. And I'm not sure whether he likes the pressure from (Ferrari president) Sergio Marchionne.
"I can imagine that Seb will switch to Mercedes in 2018 if he doesn't start going forward with Ferrari."
