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Red Bull duo Sebastian Vettel and Daniel Ricciardo believe that the reigning world champions could face a tough time at this weekend's Hungarian Grand Prix.
Vettel was third quickest during the second practice session at the Hungaroring but is wary of Red Bull's rivals being able to make greater gains ahead of Saturday's qualifying session.
Vettel was significantly faster than Williams driver Valtteri Bottas, who has claimed three consecutive podium finishes, and the German is sure that their rivals can make steps.
"I think we could be a little bit closer this weekend here, but we have seen a lot of people make big steps from Friday to Saturday - in particular Williams - so we have to be careful," Vettel said.
"I think people gain more from Friday to Saturday than we do. We hope we have a big enough step to keep the pace of today."
Team-mate Daniel Ricciardo was seventh fastest and fears that Ferrari could be the team closest to runaway leaders Mercedes this weekend.
"It was challenging, I don't think we found too many positives today," said Ricciardo.
"I think we'll find some tonight as Seb had a better day. We struggled a bit, just tried to find the pace but it just wasn't there. We went around in circles but didn't find too much.
"I think realistically Mercedes were quick again, unfortunately a lot quicker than we had hoped. Ferrari at this stage is probably the closest of the rivals."
