Ferrari warned again that they expected to be playing catch-up to champions Mercedes from the first race of the season as Formula One testing started its final week on Wednesday.
"We believe we are not the fastest car at the moment and we believe on lap time we will be behind at the start of the season," team principal Mattia Binotto told reporters at Barcelona's Circuit de Catalunya.
Alfa Romeo reserve Robert Kubica, who raced for struggling Williams last year, topped the day's timesheets with a lap of one minute 16.942 seconds in the Ferrari-powered car and on the softest C5 tyres.
Red Bull's Max Verstappen was second fastest in 1:17.347 despite a spin that halted the afternoon session with his car stranded on track.
"We won’t know how good we are until we get to Melbourne because testing isn’t about setting lap times but trying new and different things," said the Dutch 22-year-old.
"I stopped on track just before the end of the session but we’re not concerned and overall it was a positive day."
Ferrari's Sebastian Vettel was only 10th fastest overall, but did 84 laps, while team mate Charles Leclerc was 13th and completed 80 laps.
Vettel also brought out the red flags with a spin halfway through the session that left gravel across the track.
His time of 1:18.113 was Ferrari's fastest so far in testing.
"I think we have plenty of work to do. The car is a step forward... I think we know we can improve it from where we are, so time will tell," said Vettel.
Mercedes' Lewis Hamilton, chasing a seventh title this season to equal Michael Schumacher's record, was seventh after focusing on longer runs and trying out new aerodynamic components.
"It's been a good session and a good start to the second week of testing," said the Briton.
"We've just got our heads down, trying to continue increasing our mileage and our understanding of this car -- through the subtle changes with car set-up, the tyres and the aerodynamic package.
"The car didn't quite feel as good as the first week, but I'm sure that will change as the test progresses and the track conditions improve."
AlphaTauri's Pierre Gasly had been second fastest in the morning, ahead of Red Bull's Alexander Albon, who spent the first three hours in the garage as mechanics tackled a suspension problem.
Williams rookie Nicholas Latifi also halted the morning session when he stopped on track with an oil systems problem. The season starts in Australia on March 15.
(Reporting by Alan Baldwin, editing by Ken Ferris and Toby Davis)
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