Valtteri Bottas says Mercedes' vast margin over the competition in free practice for the Bahrain Grand Prix is where Williams expected the manufacturer to be in Australia earlier this month, adding it is becoming now impossible to beat their rivals.

Nico Rosberg topped both sessions around the Bahrain International Circuit, heading a Mercedes 1-2 and commanding a 1.2secs advantage over third place Jenson Button by the end of the day.

It is a margin that Williams Martini Racing driver Bottas says doesn't surprise him, suggesting it was the advantage they expected to the team to have when the season started in Australia when it was challenged by Ferrari.

"I would say the gap was surprisingly small in Melbourne and now it's maybe how it should be. Maybe Ferrari still has something up its sleeve and I'm sure we can still improve a lot from today."

Indeed, though Bottas says he doesn't begrudge Mercedes' success, he says it now seems 'impossible' to try and defeat them as F1 enters its third season of the current regulations.

"They deserve to be this far ahead. We are a bit of a smaller team and we want to win, but at this stage of the season, in normal conditions, it seems impossible. We just need to keep working and improving the car because it's still very early in the season."