British rookie Lando Norris set the fastest time in his McLaren on the first day of the final pre-season test.
The 19-year-old used the second softest tyres to pip Red Bull's Pierre Gasly by just 0.006secs before breaking down on track in Barcelona.
Gasly, using the middle of five compounds, was 0.109secs ahead of Lance Stroll's Racing Point.
Ferrari's Sebastian Vettel was a further 0.101secs back while Lewis Hamilton was 12th fastest for Mercedes.
Hamilton's team-mate Valtteri Bottas stopped on track with a loss of oil pressure and Mercedes decided to change the engine. Bottas only made it out again for the final eight minutes of the session to set the 11th fastest time.
Mercedes' upgrade was not a response to Ferrari's pace - F1 car parts take months to go from design to production to being fitted to the car.
But it did reflect the difficulty in drawing conclusions about competitiveness from pre-season testing times when development is constant and it is impossible to know in what specification the cars are running.
Mercedes said: "Part of the usual development race ahead of lights out in Oz. Going to need everything we've got. Those red cars look quick!"
At the first test, it was widely understood that the Ferrari was the fastest car, with little to choose between Mercedes and Red Bull a few tenths of a second behind.
There are three remaining days of testing this week, before the opening Australian Grand Prix in Melbourne on 14-16 March.
Fastest times, day one, final pre-season testing, Circuit de Barcelona-Catalunya
1 Lando Norris (GB) McLaren 1:17.709 ****
2 Pierre Gasly (Fra) Red Bull 1:17.715 ***
3 Lance Stroll (Can) Racing Point 1:17.824 ****
4 Sebastian Vettel (Ger) Ferrari 1:17.925 ***
5 Antonio Giovinazzi (Ita) Alfa Romeo 1:18.589 seconds ****
6 Alexander Albon (Tha) Toro Rosso 1:18.649 ****
7 Charles Leclerc (Mon) Ferrari 1:18.651 ***
8 Kevin Magnussen (Den) Haas 1:18.769 ****
9 George Russell (GB) Williams 1:19.662 *****
10 Daniel Ricciardo (Aus) Renault 1:20.107 **
11 Valtteri Bottas (Fin) Mercedes 1:20.167 **
12 Lewis Hamilton (GB) Mercedes 1:20.332 **
13 Nico Hulkenberg (Ger) Renault 1:20.348 ***
Key: * = C1 tyre (hardest); ** = C2 (second hardest); *** = C3 (middle); C4 **** = (second softest); ***** = C5 (softest)