Ferrari said Vettel’s sudden accident at turn three was caused by a mechanical problem with the front left.
Vettel, who had done 40 laps, was unhurt but went to the medical centre for routine checks.
«By the feeling there was an issue with the front left corner and from there, there wasn’t anything that I could have done to avoid the impact,» said the German.
«So after that I obviously wasn’t in control any more so more or less went straight into the barriers.»
Sainz set the best time of one minute 17.144 seconds on the soft C4 tyre and completed 130 laps, the same number as Poland’s Robert Kubica in the Williams.
«There are still a few niggles and stoppages that we wish we didn’t have, but every time we go out on track we’re a bit more reliable and we keep moving in the right direction,» commented Sainz.
«The timesheets still aren’t important. We’re only focused on our run plans and tests, rather than wasting our energy on trying to calculate where we’re at compared to the others.»
Mexican Sergio Perez was second fastest for Racing Point, on the softest C5, and Vettel third on harder and slower C3 tyres.
Mercedes were again towards the bottom of the timesheets with Valtteri Bottas eighth and five-time world champion teammate Lewis Hamilton ninth.
Between them, the Mercedes pair did 176 laps however.
«We got a lot of laps today which is a good thing, but we’ve come across a few little hurdles, so we’re just working our way through them, trying to be as diligent as possible trying to understand the causes,» said Hamilton.
«Everyone is working flat-out and I’m sure we will get there. You need days like this to be able to bring light to the things that we need to work on.»
Mercedes technical director James Allison said an afternoon race simulation had seen graining of the front tyres, and feared that could be a general feature of the year ahead.
The season starts in Australia on March 17.