Kimi Raikkonen provided the first evidence that Ferrari may be able to challenge Mercedes for the world title this year at the final pre-season test.
The Finn set the fastest time of testing so far on day three in Barcelona - and set three laps that were as fast as any Mercedes have done.
Raikkonen's lap was 0.408 seconds quicker than Felipe Massa, with Nico Hulkenberg third.
Mercedes driver Nico Rosberg was fifth, behind Toro Rosso's Max Verstappen.
Raikkonen started the day doing a couple of laps with the car fitted with a prototype version of the 'halo' head protection system that will be introduced in 2017.
Are Ferrari really as quick as Mercedes?
It is notoriously difficult to get an accurate reading of performance in testing because there are so many variables and the teams do not reveal the specification in which the cars run.
And comparisons are made even more difficult because different teams find different performance margins between the types of tyre - and their figures do not necessarily match those of supplier Pirelli.
Raikkonen and Force India's Hulkenberg used the ultra-soft tyre and Williams driver Massa the soft, which is about a second slower.
Raikkonen set a time on the soft tyre that was almost identical to one Rosberg managed on the first day of the test on Tuesday.
But track conditions vary so the two times cannot necessarily be directly compared.
It was another troubled day for the new US-based Haas team. After losing the best part of two days at the start of the test to engine trouble, Romain Grosjean had two off-track incidents on Thursday.
He blamed the first on brake-balance issues. There has not as yet been any explanation for the second, when he again appeared to lose control under braking.
The first race of the season is in Australia on 18-20 March.
1 Kimi Raikkonen (Fin) Ferrari 1:22.785
2 Felipe Massa (Brz) Williams 1:23.193
3 Nico Hulkenberg (Ger) Force India 1:23.251
4 Max Verstappen (Ned) Toro Rosso 1:23.382
5 Nico Rosberg (Ger) Mercedes 1:24.126
6 Felipe Nasr (Brz) Sauber 1:24.768
7 Fernando Alonso (Spa) McLaren 1:24.870
8 Pascal Wehrlein (Ger) Manor 1:24.913
9 Daniil Kvyat (Rus) Red Bull 1:25.141
10 Jolyon Palmer (GB) Renault 1:26.224
11 Lewis Hamilton (GB) Mercedes 1:26.488
12 Romain Grosjean (Fra) Haas 1:27.196
Raikkonen, Hulkenberg, Verstappen and Wehrlein set their best times on the ultra-soft tyre; Alonso on the super-soft; Massa, Nasr, Kvyat and Palmer on the soft and the rest on the medium