Leclerc leads the drivers' standings with 104 points after five racesCharles Leclerc headed the Mercedes drivers to set the pace in Friday practice at the Spanish Grand Prix.
The Ferrari driver's title rival Max Verstappen was fifth fastest, 0.336 seconds off the pace, but looked strong in race trim later in the session.
George Russell and Lewis Hamilton were second and third fastest, after Mercedes introduced an upgrade they hope may fix some of their problems.
Ferrari's Carlos Sainz was fourth, and Alpine's Fernando Alonso sixth.
Mercedes were just one of a number of teams to introduce significant design changes at the sixth race of the season.
Russell was just 0.117secs off the pace, and Hamilton only 0.087secs further adrift, but there will be scepticism as to whether their performance represents a genuine step forward given the difficulty of judging true performance in practice.
Mercedes also looked genuinely competitive on Friday at the last race in Miami but slipped back to their usual position as the third fastest team in qualifying and race.
Ferrari, too, have their first significant upgrade of the season, the centrepiece of which is a new floor, while Red Bull have continued their programme of taking mass out of their car to try to get it down to the minimum weight limit.
It is said that a further 4kg - equivalent to about 0.14secs in a lap time - has been shed from the Red Bull since Verstappen won the Miami Grand Prix two weeks ago, the equivalent of a little more than 0.1secs.
While the Ferrari showed promise on one lap, it struggled for race pace.
Leclerc was on average more than 0.5secs slower than Verstappen on the race-simulation runs later in the session, on the same 'medium' tyres, and complained over the radio to the team that "the tyres are going into pieces".
Behind Alonso, Red Bull's Sergio Perez was seventh, ahead of Aston Martin's Sebastian Vettel, an encouraging performance from the British team who have the single biggest upgrade of any team this weekend.
The Aston Martin features brand new bodywork that bears a striking resemblance to the Red Bull, leading to an off-track controversy.
The design has been cleared by governing body the FIA, but Red Bull are still not happy and have questioned whether some of their intellectual property has made its way to Aston Martin, who have poached several employees from the multiple champions in recent months.
McLaren and Alfa Romeo also both have major upgrades, albeit less controversial ones, but they did not have good days.
Lando Norris ran wide at the fast Turn Nine early in the second session and damaged the McLaren's floor, bringing an end to his session.
And Valtteri Bottas's Alfa Romeo broke down after he had briefly supplanted Alonso in the time sheets early in the session. The Finn ended the day down in 16th.
