The Australian clocked the quickest lap by six-tenths of a second ahead of defending three-time champion Briton Hamilton, with Rosberg third ahead of Dutch teenager Max Verstappen in the second Red Bull.
“It is definitely surprising how quick they were,” said Rosberg. “We expected them to be there, or thereabouts, but not that quick. We have some homework to do...”
Hamilton was slightly more upbeat.
He said: “I don't feel like I got a great lap so there's still time in me - not six-tenths, but still time...
“The gap is definitely closer than it looks. You'll see us within tenths of a second.
“They are very much a threat -- they're very, very fast.
“I'm not surprised because the power doesn't make a difference here and I've always known they have a good car.
“Did we expect them to be as fast as they were today? Perhaps not - may a bit closer.”
Rosberg added that he did not think that 'turning up' the power on their engines for qualifying would be enough for Mercedes to bridge the gap.
“It's very simple, today they were faster than us,” he said.
“We need to dig into it and see if we can find a big chunk of lap time.
“We can still turn up our engine, but we don't have six tenths of turning up in the engine.”
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Original source: Ricciardo's practice pace stuns rivals