Red Bull team principal Christian Horner has admitted that the race to replace Mark Webber is "probably" a straight fight between Daniel Ricciardo and Kimi Raikkonen.
Webber will quit Formula One at the end of the season and Red Bull initially revealed that the trio of Ricciardo, Raikkonen and Jean-Eric Vergne were on the shortlist to replace the Australian.
Asked by Sky Sports News if the battle for succession was now a straight race between Raikkonen and Ricciardo, Horner replied: "Essentially, probably."
Former World Champion Raikkonen remains the firm favourite to become Sebastian Vettel's team-mate next season, but Ricciardo's stock has risen quite a bit in recent weeks and Red Bull have even added him to their line-up for the Young Driver Test at Silverstone.
Horner, though, admits Raikkonen is in a different class.
"Kimi's track record speaks for itself and he's a Championship contender as we speak," he added. "We're in a fortunate position in that not only are we attracting fantastic drivers like Kimi but we've also got the opportunity to look at the drivers we have been nurturing in the likes of Daniel.
"We get paid based on where we are in the Constructors' Championship so therefore we want the fastest and best drivers we can get in the car. We'll take our time to make sure that whoever fills Mark's shoes - and they are big shoes to fill - is the right choice for the team."
Ricciardo will get his "big audition" for the Red Bull seat on Thursday afternoon when he samples the new Pirelli tyres in the RB9.
"We're very keen to have a look at Daniel as a prospect for next year," Horner said. "Obviously there is a great deal of interest in the seat, but it's an opportunity to have a look at Daniel and for him to do some of our tyre-testing work in our RBR car. It's a great opportunity for him."