Christian Horner says rivals Mercedes and Ferrari are "working as one team" as Daniel Ricciardo set the pace on the first day of testing for the 2018 Formula One season.

Ferrari have threatened to quit F1 after 2020 if owners Liberty Media do not alter their plans for the future of the sport.

And they found an ally in Mercedes team principal Toto Wolff, who said he understands why Ferrari president Sergio Marchionne has taken such a stance.

With last season's strongest two teams enjoying a growing alliance, Red Bull boss Horner says it is hard to identify difference between them.

"Mercedes and Ferrari are working as one team these days." he told Sky Sports.

"Sometimes it is difficult to tell which is which. For the last couple of years there has been a very tight relationship between Ferrari and Mercedes. They even broadcast each other's launches and things like that these days.

"We are focused on going racing, things that we can control, there are new owners in Formula 1 in Liberty and they have to decide what they want Formula 1 to look like moving forwards.

"I'm sure they will lay out their roadmap and plan during the course of this season and we will go from there.

"But of course there is going to be a lot of posturing and positioning and you can see that has already started in certain quarters."

Ricciardo was quickest in both sessions at the Circuit de Catalunya in Barcelona on Monday and was the only driver to complete 100 laps, edging out Valtteri Bottas' Mercedes and Ferrari's Kimi Raikkonen.