"It is unfair to put so much pressure on Leclerc", world champion Hamilton said in Barcelona.
"I think we feel at this point they're a bit ahead, but obviously it's impossible to make detailed calculations". "We don't know how much the others are sandbagging so we'll see at the first race".
The highly-regarded 21-year-old Monegasque starts his first season with the Italian giants after leaving Sauber to replace Kimi Raikkonen, who was the last man to win a drivers' world title with Ferrari in 2007.
"The performance has no sense for now because it's testing, they are not pushing and we are not either", Leclerc said, as quoted by Autosport.
Ferrari driver Sebastian Vettel threatened to mount a title challenge past year but the four-time champion's performance levels dropped during the second half of the season, allowing Hamilton to ease to his fourth triumph in five seasons.
Vettel sang his praises after day one, stating the team had been close to "perfection", but Leclerc is eager for Ferrari to push its testing programme further.
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Despite finishing behind both Mercedes' in Thursday's running, Leclerc is adamant Ferrari's rivals are holding back.
"This looks like it is going to be the most challenging year of our partnership so I'm super excited".
Mercedes, chasing a record sixth successive title double this season, have said they intend to focus more on speed and performance at next week's second test after mainly concentrating on reliability until now.
Ferrari can boast of 15 drivers' crowns and 16 constructors' titles, the last of which came in 2008. "We are going to face our toughest challenge but I feel like I have the right soldiers behind me".
"I think there is a lot of potential, but it's not yet quite there", Bottas said. Williams, which started testing later than the other teams because of delays with its 2019 vehicle, ran only 88 laps in total, the fewest among all teams.
However, Hamilton did acknowledge that his side have some work to do before the season kicks off in Australia on March 17.
"In the team, (there is a) kind of. excitement to discover more about the vehicle and really try to improve it because at this point it looks like we are not miles ahead of everyone".