Australian driver Daniel Ricciardo said on Thursday that his switch from Toro Rosso to Red Bull was "not a done deal".
He said that in his own eyes the seat alongside world champion Sebastian Vettel remained up for grabs in 2014 when fellow-Australian Mark Webber leaves Formula One to go sportscar racing.
Ricciardo became favourite for the slot when talks between Red Bull and Kimi Raikkonen ended earlier this week.
Red Bull team chief Christian Horner had already ruled out Ricciardo's Toro Rosso team-mate, Jean-Eric Vergne.
"I'm not going to assume anything. For me there is still a seat up for grabs," said Ricciardo.
"Obviously there is a bit of excitement around, but there is nothing further to say from what I knew in Budapest. I think I've learned that until something is signed, sealed and delivered nothing is ever concrete.
"There's still my team-mate Jean-Eric, who as far as I'm concerned isn't out of the picture, and a lot of other drivers on the grid with pretty good credentials.
"Until something comes from that side (Red Bull), I'm not going to start celebrating or getting my hopes up."