The returning Robert Kubica completed 48 laps in his first pre-season test as a full-time F1 driver since 2010, with Williams bidding to make up for lost time after missing the first two and a half days.
"At the moment I don't really hold any worry about anything", said the 34-year-old Briton, who conceded that Ferrari did appear to have produced a better package than last season.
Ferrari driver Sebastian Vettel threatened to mount a title challenge previous 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.
But amid suggestions that Mercedes might be as much as 0.5s per lap off the pace of Ferrari on a like-for-like run, Hamilton said he was not yet panicking about the pace of the red auto. That is the same for all of us when you come to a new year, you set new heights and new targets.
Valtteri Bottas says he is approaching the 2019 Formula 1 campaign with the feeling that he "hasn't achieved anything" in the championship, as he bids to improve on a disappointing 2018.
Mercedes, victor of both titles for the past five years, has also been pounding out the mileage without setting any eye-catching times.
Nishikori, however, will be out to stop the Swiss ace and claim the $565,000 winner's prize and his second title of the year having won the Brisbane International last month.
"It is unfair to put so much pressure on Leclerc", world champion Hamilton said in Barcelona.
Mercedes also ran 12 more laps than Ferrari during the four days of testing.
"I just want to make sure that this year I'll do everything I can to achieve those targets and I'm basically willing to do whatever it takes to make sure I achieve those goals".
"I think we have a strong auto". The other teams are sandbagging.
"There were some corners that were good, some corners we would have balance issues and other corners we would have massive balance issues".
Hamilton's bid for title number six begins at the Australian Grand Prix in Melbourne on March 17.
"I sense a bit of an excitement in the team, to discover more about the vehicle and we're really, really trying to improve it, because at this moment it looks like we're not miles ahead of everyone".