McLaren had looked set to top the standings for a second successive day in Barcelona, but in a reversal of Wednesday's proceedings Daniel Ricciardo was pushed off top spot by Leclerc's 1:19.689, which was just over a tenth of a second off Lando Norris' top time from the opening day.

The Australian ultimately ended up third, as Pierre Gasly posted the second-fastest time of the day for AlphaTauri, with Mercedes' new driver George Russell fourth.

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With Lewis Hamilton 16th - at the very bottom of the timesheets - Mercedes appear to be taking it easy at testing, although Russell suggested the world champions were currently behind Ferrari and McLaren, who have taken turns in setting fastest times for much of the opening two days of the sport's new era.

"Some teams, a red team and an orange team in particular, look very, very competitive," said Russell.

"[Mercedes are] certainly not ahead, pretty sure of that. We all know that we are on different programmes but we definitely know from the average of all the different runs, we're behind them at the moment. So let's wait and see.

"The championship's not won in Barcelona pre-season testing, but it's certainly been an intriguing two days for now."

Red Bull, meanwhile, suffered their first reliability issue of the season through Sergio Perez's gearbox issue as the Mexican driver stopped on track.

After a surprising lack of stoppages on Wednesday, it was the first red flag of testing - with Nikita Mazepin later interrupting the afternoon action with a damaged fuel pump on the Haas.

Russell completed an encouraging 65 laps, after Hamilton experienced a limited morning session during which he was able to complete just 40.

However, Mercedes' total of 105 laps was still significantly greater than the 78 managed by rivals Red Bull, with Perez missing about two hours' worth of running time.

Perez did produce a solid stint upon his return, with a late time propelling him to seventh, and with 147 Max Verstappen laps in the bank from day one, there appears to be no reason for major concern in the Red Bull garage.

Having announced between the two sessions that he would not compete in this year's Russian Grand Prix, after suggesting it should be cancelled following the country's invasion of Ukraine, Sebastian Vettel set the sixth fastest time while completing 74 laps for Aston Martin. His team-mate Lance Stroll was 12th after managing 55 laps.

Carlos Sainz was fifth as Ferrari had another encouraging day, totalling the most laps for any team on the day with 150 circuits completed by the Spaniard and Leclerc. Gasly was just short of the mark with 147 laps, while Ricciardo managed 126.

Ferrari team principal Mattia Binotto attempted to play down his outfit's strong start, insisting they are "outsiders not favourites".

"I think it's very early days," Binotto said during the lunchtime press conference in Barcelona. "It's always great to have good consistency when running because it's time to learn these new cars.

"When you're doing a lot of laps, you are certainly collecting data. So far I'm pleased by the fact that yesterday we did many laps, and this morning too, which is important. Regarding the pace, it's very early to judge. What I can see is that we are all very close, which was one of the objectives of the new regulations."

Despite Mazepin's stop on track, there were encouraging signs for Haas as the Russian driver moved up to eighth late in the day, while Alfa Romeo also showed signs of improvement as Chinese rookie Guanyu Zhou completed 71 laps in the afternoon session.

Williams completed a solid day with Alex Albon and Nicholas Latifi combining for 108 laps and finishing ninth and eleventh, respectively, while Esteban Ocon was 14th after completing 125 laps as the sole Alpine driver.

What is 'porpoising'?

There was much talk of 'porpoising' on day two in Catalunya, with several teams struggling to avoid an issue that appears to have returned following the sports biggest rules overhaul in 40 years.

Teams are testing for three days in Barcelona with new rules that are aimed at improving the show and battles throughout the field, with emphasis on cars being able to follow each other more easily.

'Porpoising' - a violent bouncing of the suspension at high speeds caused by aerodynamics, which recreates a similar motion to that of a porpoise swimming, was understood to be one of the factors that limited Hamilton to just 40 laps in the morning session.

"It's when the car imitates the kind of swimming motion of a porpoise," Sky Sports News reporter Craig Slater explained from the pitlane.

"So it kind of pops up when the air flow detaches - and the more a car bounces around, the more performance you lose and the Mercedes has been a little bit prone to that.

"So it's getting the right tune between the floor and the front wing, so the air flow doesn't detach, because it's the air flow around the car which keeps it absolutely stuck to the ground."