Valtteri Bottas has warned Mercedes is 'in a league of its own' ahead of the 2016 Formula 1 World Championship as the team flexed its pace on the first day of the second pre-season test.

With teams expected to bring more performance runs into their programmes in this final four-day test, Williams trialled the ultra-soft for the first time, Bottas setting the second quickest time with a 1min 23.229secs.

Four tenths off Sebastian Vettel's ultra-soft effort in the Ferrari last week, crucially Bottas was also slower than Mercedes' Nico Rosberg today by two tenths, albeit with the German using soft rubber. Asked whether he felt that it is Mercedes showing what it is potentially capable of, the Finn gave a blunt assessment.

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"They are in their own league," he said. "I think that is a fact at this stage in the season. At the moment it looks like they are in a really strong position. We are doing our best [to catch them]. We're really trying to chase them."

Despite this, Bottas was encouraged by the FW38's reaction to more performance orientated runs over the course of the day, saying there has been a good step forward over last year's car.

"I've driven a car before that when you took the fuel out and out you on a softer tyre nothing happened with the laptime. It was a good sign today that we're starting to change the programme and do more and more performance work. The lap times were coming down nicely, and we know we still have plenty more to come."

Though he considers Mercedes to be out of reach, Bottas feels Williams is in the midst of the chasing pack with Ferrari and Red Bull.

"Mercedes is looking very strong, also Ferrari and Red Bull. We are there or thereabouts with Ferrari and Red Bull, but it is difficult to say in detail."