Having fallen foul of a reliability glitch, Nico Rosberg says he is optimistic but not confident of Merc being "bullet proof" for Melbourne.
The German was in action on Day One at the Sakhir circuit in Bahrain and completed 89 laps with a best time of 1:36.624.
His day, though, was not without problems as an issue in the afternoon curtailed his running.
"I managed to do a couple of good laps, but in the afternoon we had a few reliability problems," he told to Autosport.
"It just shows everyone is fully on the limit.
"OK we're looking a little better on reliability than some others, but even we're on the limit and having problems.
"To get everything ready and bullet proof for Melbourne is really tough.
"I'm optimistic, but confident? Speed doesn't help if you don't get to the end of the race.
"Everything needs to fit together; we're not there yet and there's just a few days left. I hope we're ready for Melbourne; it's going to be close."
As for his day's running, the 28-year-old revealed that Mercedes have started to push harder with their W05 which is why problems have begun to appear.
"We had a lot of new things that took a while to get going.
"We're going into the details now, which is a good thing, but when you do that you find new problems.
"Unfortunately a problem stopped me running before the end of the day, but we're still going in the right direction.
"The heat [on day one] was more representative of the first couple of races too, and that allows you to discover problems maybe you wouldn't if it is a bit cooler.
"We're in a decent position starting off now, but I don't feel we're that far ahead. Everyone has problems and we do too."