Nico Rosberg reckons Mercedes may not have it all their own way as Ferrari could be "very close" this season.
While the Scuderia's drivers dominated the timesheets in the opening test, it was Rosberg who came to the fore on Tuesday at the Circuit de Catalunya.
Putting the soft tyres on his W07 for the first time, he shot up into P1 with a 1:23.022. That time was only 0.2s shy of Sebastian Vettel's best from last week which he had set on the ultra-softs.
"It's difficult to say for now, it's very close between Ferrari and us I think, and maybe some other team might be there as well," Rosberg told ESPNF1.
"We were doing qualifying practice this morning as it's important to practice that as well, and that's all the more enjoyable to take the car to the limit and really push."
Pressed specifically on the threat of Ferrari, he added: "It's close out there between the two teams I think.
"It's difficult to know where [we are] exactly but they've done a great job so we need to really give everything because they are very close and a big threat. But of course we are confident and we are ready for a battle, for sure."
As for his morning's work, Rosberg covered 82 laps and was happy to finally take some fuel out of his Mercedes and see what it could really do.
"It was a decent morning for sure, making progress," he said. "We took some fuel out for once which was nice for me to get in to it to be able to push some more, so we put down some lap times and it was a good feeling."