Red Bull team principal Christian Horner believes Mercedes has not yet shown its hand but suggested Ferrari is the team to beat at next weekend's Bahrain Grand Prix.
The Scuderia impressed throughout the two pre-season tests with heavy mileage coupled with competitive pace, leading rivals to earmark Ferrari as a favourite in the early stages of the season.
By contrast, Mercedes seemingly struggled to get over understeering issues and despite logging the most laps of any team in Bahrain, George Russell's best lap of the week was good enough for only fifth fastest.
Max Verstappen set the fastest time of the week on the final day with a lap good enough to be a second clear of second-placed Mick Schumacher but speaking before the time was set, when asked who would be favourite for the first race, Horner replied: "It is just impossible to tell.
"What you can say, it is only going to be next Saturday you will get that snapshot when the fuel comes out, people run as close to the weight limit as they can and we see how competitive they are.
"I think that is fascinating. We have a complete clean sheet of paper, I don't think Mercedes have fully shown their hand yet.
"Ferrari look the form team, you would have to say at the moment. Hopefully, we can be in there as well, it looks like Mclaren have a strong car, Alpine have been quick so it is impossible to predict."
Reaffirming his stance that Ferrari is on top after testing, Horner added: "For me, [it is] the car that looks the most settled on circuit at the moment.
"I think they have had a very strong testing period so far, both in Barcelona and in Bahrain so far. They have looked extremely competitive for everyone on track.
"You have to remember these cars are still very immature. The rate of development will be fast and intense and I expect that to change and Mercedes will be a huge factor in this championship, I have no doubt."