Sebastian Vettel believes Red Bull and Toro Rosso could join the fight behind Mercedes in this weekend's Malaysian Grand Prix.
Ferrari was still the closest challenger to the German squad, with Kimi Raikkonen finishing in second behind Lewis Hamilton in the afternoon practice session on Friday.
Vettel, who finished third behind the Mercedes in Australia, was down in seventh place behind the Williams drivers and the Red Bull of Daniil Kvyat.
Max Verstappen was eighth in the fastest Toro Rosso.
"We'll see. I think it will be close between us and Williams," said Vettel after second practice. "Probably Red Bull as well and the Toro Rossos.
"It's difficult to say, it's a mixed bag. But at the front Mercedes is strong."
Vettel spun into the gravel during second practice, and he admitted he was just pushing the limits.
He added: "It's quite hot which makes it difficult for all the cars to work with the tyres, but all in all I think it was a decent day.
"I'm not entirely happy with myself, I had a spin, but I think you have to try and if there's a day you choose to try then probably today is the day."