Mercedes cannot be confident they will start the new 2017 F1 season at the front, according to Valtteri Bottas.
Ferrari set the fastest time seen at winter testing this year when Sebastian Vettel beat Bottas' previous benchmark on the penultimate day to register the quickest lap at the Circuit de Catalunya since 2009.
Bottas' team-mate Lewis Hamilton, who has previously described Ferrari as the favourites for 2017, was three tenths slower than Vettel on the same compound.
"We are definitely not confident that we are in front," said Bottas. "We are not relaxing, we are not in a comfortable situation at all. We do feel that the other teams have made good progress over the winter.
"Ferrari are looking very strong, Red Bull can always surprise, and other teams can be strong. But Ferrari look solid, strong and fast - and who knows what people will bring to Melbourne.
"We should not underestimate them at all. We're not saying we're number one, we're just working flat out."
Ferrari are the only team yet to lap below the 1min20sec mark on soft tyres this winter although Bottas' benchmark time on supersofts remains marginally faster than Vettel's personal best on Pirelli's red-marked rubber.
But it was Vettel's table-topping effort on ultrasofts which was inevitably the major talking point at Barcelona on Thursday - especially after the Ferrari eased off in the final sector.
"A very fast time," Bottas acknowledged. "It's difficult to know what type of things they were running and how much they were taking out of their car but a respectable time and very quick. Definitely, we shouldn't underestimate them."
Bottas has replaced the retired F1 champion Nico Rosberg alongside Hamilton for 2017 and described his first impressions of measuring up against the driver considered to be the fastest on the grid as an "interesting" experience.
"We've been running quite different programmes but definitely on occasion there has been good data to compare to which has been very valuable," he said. "So far it looks quite close but it is only testing and I'm not sure that either of us have been doing qualy-type laps or in a race situation. But so far from what I've seen I've made good progress."
Martin Brundle's Testing verdict
Standing trackside for his first close-up look at F1's new generation of faster cars, Sky F1's Martin Brundle said they appeared more "positive" and were allowing drivers to carry "a little bit of extra speed everywhere".
The Sky F1 pundit believes Mercedes remain the team to beat, but Ferrari and Red Bull may have closed the gap from the end of 2016.
"Talking to a few people in the paddock it seems that Mercedes might have the edge," Brundle told Sky Sports News HQ.
"But Ferrari and Red Bull are quite close behind them, so that bodes well. Williams are pretty strong as well, and the midfield is very strong. My concern was maybe Mercedes might run and hide, but at the moment at least it looks like they're in touch."
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