The Monegasque is preparing for his graduation to Ferrari in 2019 after ending his rookie season with Sauber in last weekend’s Abu Dhabi Grand Prix.
LeClerc has driven for Ferrari before, including a run at Monza earlier this year, but predicted his first test as an official race driver would be an “emotional” and “special” experience.
He kicked it off by setting the fastest time across the opening four hours of Wednesday’s running, which will continue for a further five hours until 6pm local time.
LeClerc moved to the top of the times in the second hour with a 1m36.938s, then shaved two hundredths off that later in the morning, leaving him just over a tenth slower than his 2019 teammate Vettel’s benchmark on Tuesday.
The 21-year-old then went even quicker shortly after midday, setting a 1m36.559s – almost three tenths clear of Vettel’s best.
Like LeClerc, Pierre Gasly is getting a taste of one of F1’s current big-three teams in the test as he appeared for Red Bull ahead of his own 2019 move.
Gasly, who last drove a Red Bull in testing in August 2017, set the second-fastest time, 1.4s slower than LeClerc after his own improvement in the fourth hour.
Unconfirmed-but-expected Racing Point Force India driver Lance Stroll reappeared for the team after wearing pink for the first time on Tuesday.
Stroll set the third-best time, joining the top two in improving after midday and lapping just 0.07s slower than Gasly.
Valtteri Bottas continued for a second day of testing at Mercedes and was fourth quickest after the first four hours.
Daniil Kvyat got back behind the wheel of a Toro Rosso for the first time since being dropped after the 2017 United States Grand Prix.
The recalled Russian completed the top five, 2.3s slower than LeClerc.
His compatriot Artem Markelov was next up for Renault, while Louis Deletraz made his F1 testing bow with Haas to run seventh fastest with a best time of 1m40.153s set in the second hour.
That put him two tenths clear of new McLaren man Carlos Sainz, taking part in his maiden outing with the Woking team.
Antonio Giovinazzi took over from Kimi Raikkonen at Sauber and went eighth fastest, ahead of Renault’s Artem Markelov.
Robert Kubica reappeared for Williams but did not representative times in the morning. He will hand over to 2019 teammate George Russell for the rest of the day.