Valtteri Bottas says Mercedes has been in a league of its own during testing and is hoping Williams will challenge the likes of Ferrari and Red Bull to be the best of the rest at the first race.
Mercedes hinted at its true potential on Tuesday with a qualifying-style lap on soft tyres that was 0.207s quicker than Bottas' best effort on the faster ultra-soft tyres.
"They are in their own league," the Williams driver admitted. "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. At least where we are now it looks like it's going to be quite tricky for anyone to challenge them at the beginning of the season. That is a fact at the moment. "Let's see. I'm sure we'll still progress a lot from these tests and there is more to come."
Bottas' lap on Tuesday was over 0.4s off Sebastian Vettel's best time last week on the same ultra-soft tyres, but he still thinks Williams can be in the mix with Ferrari.
"It is always difficult to say. We are there or thereabouts with Ferrari and Red Bull, but it is difficult to say in detail. Today was nice to see when we pushed a bit more with a programme to see the lap time was coming down nicely, which is a good sign."
Asked if it is a shame for the fans to see Mercedes so far ahead, Bottas added: "Yeah, I know. We are doing our best.
"We're really trying to chase them. It's a long season, and who knows. We hopefully have plenty of upgrades to come from Melbourne, and hopefully they will work well. Don't lose your faith yet."