Sebastian Vettel crashed his Ferrari on the second day of the final pre-season test in Barcelona, severely curtailing the team's running.
Vettel lost control after what the team described as "a mechanical problem" at the 150mph Turn Three and hit the barriers, causing extensive damage.
"From the feeling in the car it was an issue with the front-left corner," the German said.
McLaren's Carlos Sainz set the fastest time, from Racing Point's Sergio Perez.
Mercedes' Valtteri Bottas and Lewis Hamilton were eighth and ninth fastest respectively.
Vettel, who was third fastest, said the team were having difficulty establishing what had caused the incident.
"It would be a lot easier if there was a lot of run-off and the car didn't hit anything right after because now it is fairly damaged because of the hit. So we have to be careful working out what happened," he said.
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The team managed to get the car out again only for a brief run with Vettel's team-mate Charles Leclerc at the wheel with seven minutes of the day remaining, a delay of nearly five hours.
It was the first major blip in what had until then been a promising pre-season period for Ferrari, who it is widely accepted have the fastest car in the field at this stage.
But Vettel said the team were positive despite the setback.
"Overall the car is strong, it feels good," he added. "We have been exploring a lot of things since day one and it has been very productive. Overall, I am still positive about the car and the feeling inside the car."
Sainz made it two days in a row for McLaren to top the times, after team-mate Lando Norris did so on Tuesday, with the fastest time of pre-season testing so far.
And it coincided with McLaren's announcement that their former driver, the two-time champion Fernando Alonso, was staying on board as a technical adviser and ambassador.
The Spaniard will test the McLaren this year to aid the development of this year's and next year's cars, and will be involved in an advisory capacity at many race weekends, whether at the track, or remotely, depending on his racing commitments elsewhere.
Alonso has a contract with Toyota in the World Endurance Championship until the Le Mans 24 Hours in June and is racing for McLaren in the Indianapolis 500 in May.
Sainz used the second-softest tyre compound to do his fastest lap, while Vettel was on the harder and slower middle compound as he went about a second slower.
Alonso said the team were content with progress with their new car so far, at the start of a season in which they want to show significant progress after a disappointing 2018.
"There are a couple of areas they need to revise and keep developing to improve performance, some others are surprisingly good," Alonso said. "Generally it has been a very positive two weeks. It is not perfect yet but the direction seems the right one."
Mercedes continue to operate under the radar in pre-season testing, but it is almost impossible to predict competitiveness accurately from these days in Barcelona because the teams do not reveal the specifications in which they are running their cars.
Fuel loads, tyres, settings and engine modes can all make significant differences to car performance.
There are two remaining days of testing this week, before the season-opening Australian Grand Prix in Melbourne on 14-16 March.
Fastest times, day two, final pre-season testing, Circuit de Barcelona-Catalunya
1 Carlos Sainz (Spa) McLaren 1:17.144 ****
2 Sergio Perez (Mex) Racing Point 1:17.842 ****
3 Sebastian Vettel (Ger) Ferrari 1:18.195 ***
4 Kimi Raikkonen (Fin) Alfa Romeo 1:18.209 seconds ****
5 Romain Grosjean (Fra) Haas 1:18.330 *****
6 Max Verstappen (Ned) Red Bull 1:18.395 ***
7 Daniil Kvyat (Rus) Toro Rosso 1:18.682 ***
8 Valtteri Bottas (Fin) Mercedes 1:18.941 ***
9 Lewis Hamilton (GB) Mercedes 1:18.943 ***
10 Nico Hulkenberg (Ger) Renault 1:19.056 ***
11 Robert Kubica (Pol) Williams 1:19.367 ****
12 Daniel Ricciardo (Aus) Renault 1:22.597 *
Key: * = C1 tyre (hardest); ** = C2 (second hardest); *** = C3 (middle); C4 **** = (second softest); ***** = C5 (softest)