Mercedes fear rivals Ferrari hold an early advantage as Formula One's opening winter test draws to a close.
World champion Lewis Hamilton posted his fastest time of the week on Thursday, as Mercedes finally turned on the speed in the concluding day of running.
But the 34-year-old British driver, nor his Mercedes team-mate Valtteri Bottas, have failed to trouble the top of the time charts at Barcelona's Circuit de Catalunya.
In contrast, Ferrari have flown out of the blocks, with Sebastian Vettel and his new team-mate Charles Leclerc quickest on the opening two days.
"Ferrari do seem very strong," said Bottas. "We feel at this point they're going to be a bit ahead.
"No matter which kind of fuel load or engine modes they're running, they have been quick, both in their short runs and long runs, too.
"With the new rules it is going to be a big development race, but we do need to try and make our car better and quicker."
Hamilton will head to Turin for a Mercedes sponsorship event on Friday, before returning to Barcelona next week for the final four days of testing. He opens his championship defence in Melbourne on March 17.
On the final morning, novice Alexander Albon posted the fastest lap of the winter so far, with a time of one minute 17.637 seconds.
The London-born Toro Rosso driver finished ahead of Daniel Ricciardo's Renault, with Hamilton third, a third of a second adrift.
British teenager Lando Norris slipped off the track and into the gravel in his McLaren, but still managed nearly half-a-century of laps.
The returning Robert Kubica completed 48 laps in his first pre-season test as a full-time F1 driver since 2010, with Williams bidding to make up for lost time after missing the first two and a half days. He finished last, four seconds off the pace.
After lunch, Renault driver Nico Hulkenberg usurped Albon with a lap time of one minute and 17.397 sec.
Bottas meanwhile, edged out team-mate Hamilton to relegate the Englishman to fifth.
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