AP Photo / Manu Fernandez.

Barcelona - Ferrari is looking stronger than Mercedes in testing and the Formula One championship-winning team has work to do before next month's season-opening race in Australia, Valtteri Bottas said on Thursday.

The Finn, team-mate to five-times world champion Lewis Hamilton at Mercedes, gave his assessment on the final day of the first four-day pre-season test at Barcelona's Circuit de Catalunya.

Ferrari was fastest on the first two days and the Italians also covered an impressive amount of mileage in the new SF90 car while Mercedes has been working through its programme without setting any eye-catching times.

"They do seem very strong," Bottas said of Ferrari, Mercedes' closest rivals last year.

"No matter which kind of fuel load or engine modes they are running, whatever you try to correct that for, in any case they are quick on short runs and long runs.

"I think we feel at this point they are maybe a bit ahead, but obviously it's impossible to make detailed calculations. It is very early days. With the new rules its going to be a big development race."

Bottas said the cars would be very different by the time of the Melbourne race on March 17 and Mercedes had new parts coming before then.

The Finn warned, however, that even with the upgrades Mercedes still needed to make the car better and quicker, with work to do on the balance.

"It feels a little bit different (to last year), at least in these (winter) conditions," he said. "It seems to be again, with the tyres, quite a narrow window to get them to work.

"It seems like balance-wise a little bit on a knife-edge at the moment. But there's nothing fundamentally wrong, we feel. I think there is definitely a lot of potential but it is not yet quite there.

"In the team, (there is a) kind of ... excitement to discover more about the car and really try to improve it because at this point it looks like we are not miles ahead of everyone. It seems like Ferrari are in a better place."

Mercedes has said it intends to focus more on speed and performance at next week's second test after mainly concentrating on reliability until now.

Hamilton also recognised Ferrari was quick when he spoke to reporters earlier in the week but said that was the case in testing last year and Mercedes prevailed in the end.

Mercedes is chasing a record-equalling sixth successive constructors' championship this year and also a record sixth successive title double.

Ferrari last won a constructors' championship in 2008, the year of Hamilton's first drivers' title with McLaren.