As the 2019 winter tests approach their conclusion, Valtteri Bottas has admitted again that Ferrari continues to lead the pecking order.

Charles Leclerc set a blistering fastest time on Thursday, the penultimate test day in Barcelona, and also impressed on a so-called 'long run'.

"When we calculate everything, then Ferrari would have beaten us to the finish of a race by five seconds," Mercedes team boss Toto Wolff admitted to Auto Motor und Sport.

"That's the bitter pill. But the good news is that we were faster than Red Bull on both of their drivers' long runs," he added.

But that may not be the full story. One analysis in the paddock on Thursday is that Mercedes' 2019 car could be like its 2017 predecessor - a "diva".

In other words, even some midfield teams like Alfa Romeo reportedly believe that they are faster than Mercedes on occasions.

Bottas said: "The improvements we made for the second test have made the car better. Whether it's enough to be competitive in Australia remains a question mark.

"Ferrari is still very strong. Things can change for the start of the season, but when I look at our situation, it's clear that we can do more," the Finn added.

As for the claim that Mercedes' 2019 car might once again be a temperamental "diva" a la 2017, Bottas answered: "If that's how it is, it doesn't scare me.

"We are still right at the beginning with this car. As we progress, I'm not worried."

Bottas also doesn't deny that Red Bull might be slightly ahead of Mercedes as the circus prepares to move on to Australia.

"I don't think the difference between us and them (Red Bull) is massive," he said. "But Ferrari is ahead of us. I also think the whole field has moved closer together."

The big question now is whether Mercedes will stay the course with its current basic aerodynamic concept, or change direction completely to 'copy' Ferrari.

"I think Red Bull is asking the same question," team boss Wolff said.

He said the problem with that approach is that it could seriously compromise Mercedes at the start of the season.

"You would have to write off the first races because we would have to freeze our concept until the other one is ready," said Wolff.