Williams driver Valtteri Bottas has said he believes his team can challenge Ferrari and Red Bull, but noted that Mercedes appears "in a league of its own".
Bottas, who will kick off his fourth season as a Williams race driver in Melbourne, was runner-up to Nico Rosberg in Tuesday's test.
"Difficult to say in detail, but today it was nice to push a bit more and see that the lap time was coming down nicely.
"So it was good, and we know we still have plenty more to come. But whether we’ll be up there in Melbourne, I don’t know yet."
While Bottas' best time of the day, done with ultrasoft tyres, did put the FW38 a clear second, he still came up short of Rosberg's benchmark despite the German setting his time on softs.
Asked about Mercedes, Bottas said: "They are in their own league at this point - that is the fact. I think it will be tricky for anyone to really challenge them in the beginning.
"We are doing our best to chase them, it’s a long season, and we still have upgrades to come for Melbourne and hopefully they’ll work well. Don’t lose faith yet."
Additional reporting by Jamie Klein