Seven-time world champion Lewis Hamilton admitted on Friday that the odds of Mercedes keeping pace with the Red Bulls in Australia were slim and that matching his fifth place in Saudi Arabia was a more attainable goal.

The Briton came a surprise second in the opening practice at Albert Park behind Red Bull's Max Verstappen, before slumping to 13th in a rain-affected second session.

He blamed changes made to the car between the two-hour runs for the gaping difference.

The car "felt a little bit better this morning, we made some changes, they didn't work, so we'll revise those tonight," he said.

"Then it rained obviously so it wasn't the best of sessions. You don't really learn a huge amount when it's between (conditions), so just keep the car safe."

He learned enough to know that come race day on Sunday, Mercedes will again be playing catch-up to Verstappen and his Red Bull teammate Sergio Perez.

On the high-speed, power-friendly Jeddah track a fortnight ago, Mercedes showed signs of life, with Hamilton fifth and his teammate George Russell fourth.

It was a clear improvement from the season-opening race in Bahrain and they bettered the Ferraris but were still behind Fernando Alonso's Aston Martin and the victorious Red Bull team.

Hamilton said another fifth-placed finish would be a good result for him in Australia.

"We won't be competing against the Red Bulls," he said. "It will just be about trying to see if we can get up as high as possible. I think we've got pace to be around fifth, same as the last race."

Russell, who was fourth in the second practice, was more optimistic, saying it "wasn't too bad a day".

"We've been making good steps with the car on Fridays. We've been changing a few things and getting the car in a nice window as the weekend progresses," he added.

"That's quite satisfying from the driver's seat, knowing that the car feels better and as a team we have contributed towards that."