Both Sainz and Leclerc had spins during the say – Sainz’s at Turn One and Leclerc’s at the final corner – just as they were starting flying laps.

Mercedes drivers George Russell and Andrea Kimi Antonelli were fourth and fifth fastest.

McLaren’s Oscar Piastri and Lando Norris were 13th and 14th fastest.

Pre-season testing is a notoriously unreliable indicator of actual competitive performance because factors such as fuel loads and engine modes are not revealed by the teams and have a major impact on performance.

And despite Sainz’s fastest time, Williams team principal James Vowles underlined that his team were not fast enough to compete at the front this year.

“The top four are the top four,” Vowles said. “McLaren, Ferrari, Red Bull and Mercedes have all done good work over the winter.

“The midfield is going to be very tight and if I’m getting it right, 0.1secs separates a lot of us.”

Red Bull’s world champion Max Verstappen did not drive while team-mate Liam Lawson was in the car all day, ending up seventh fastest; Verstappen will drive on the final day on Friday.

Leclerc said before running in the afternoon: “It’s only testing. Everyone is hiding their true performance but the feeling is pretty good. No bad surprises but too early to judge performances.”

Norris said in the news conference before driving in the second session: “We have just tried to make the car quicker all round. Add more load. If we want to improve anything at the moment, it’s the rear but the rest of it feels correct and feels in the same ball park.

“I don’t think we are expecting to be a big step ahead of everyone. We are expecting to be close to Red Bull and Ferrari. If we are there from the off, that’s the main thing.”

Neither McLaren driver did any short performance runs during Thursday’s running.