Jenson Button admitted that he was surprised at how much faster than everybody else Mercedes was during Friday's first day of practice in Australia.
Mercedes pair Lewis Hamilton and Nico Rosberg topped the time sheets on the first day of practice ahead of the season's first Grand Prix, while Fernando Alonso in a Ferrari was third fastest, albeit about half a second off the pace.
Button was second quickest in the first sessions but dropped down to fifth overall later in the day, but was surprised that rival teams Williams was so far off the pace, despite being powered by Mercedes engines as well.
"We were two seconds off them [Mercedes] in the test and we're over a second off them [in practice] in the long runs," Button told Autosport after FP2.
"I think everyone's about a second off them, they're very quick.
"I don't think our outright pace is as good as it looks in practice but we're OK; our balance still isn't there.
"We're nowhere near the quickest, [but] it's a little bit surprising that [Mercedes is also] much quicker than everyone else, including the Williams - which looks a bit of a handful to drive in high-speed corners which is a surprise as well."
Button added that fuel reservation could be a key aspect during Sunday's race and that Mercedes' raw speed could help them in that regard.
"I don't know if other people [are having to] save fuel a lot but it seems we have to in the race," the former World Drivers' Champion told Autosport.
"This is a very difficult circuit for fuel saving and using 100kg, so this whole race for us is going to be tricky with how much we have to back off.
"This is probably one of the worst circuits for fuel usage so it's going to be a tough one on Sunday.
"Outright pace is always [desirable], because if you have a quick car you can save fuel more easily - just back off more.
"At the moment we're reasonably reliable, engine's working really well, and our pace is alright.
"We're not as quick as the quickest cars but then we're a lot quicker than the slowest cars, so we're not in too bad a shape."