Valtteri Bottas admits that he was surprised about the speed of Mercedes at Barcelona today after Nico Rosberg left even Williams trailing behind.
Rosberg was 1.2s faster than Bottas today, and 0.7s faster than the previous overall best time set by Felipe Massa in the FW37 yesterday.
"It was a good day for us, again a completely trouble-free day, and that's always nice," said the Finn.
"It was more performance work today, which was nice, nice to get a little bit shorter runs and really try to squeeze a bit out of the lap time.
"It felt OK, but a little bit surprised about the speed of Mercedes."
He added: "It's difficult to say exactly how much they are in front, but they are in front. I think it's the team to beat."
While pleased with progress Bottas says that Williams can still do more before Australia.
"We're really looking forward to it, we know that we can be competitive, but we still have work to do.
"There's still two days left of testing, and we're still going to have a really busy plan. We're ready to race, but there's things we can still do better, and hopefully we can sort some things out in the next few days."