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The 34-year-old Briton enjoyed a near flawless run to secure his fifth drivers' title in 2018.
He won 11 of the 21 races to beat Ferrari's Sebastian Vettel with two races to spare, and he expects the Italian marque to once again keep him on his toes.
"Ferrari are very strong," Hamilton said in Barcelona on the third day of pre-season testing on Wednesday with Daniil Kvyat in a Toro Rosso topping the times from the Ferrari of Kimi Raikkonen.
"It appears they have a better package than last year, which means it will be a bigger challenge for us," he suggested.
"In the last few years, Ferrari has looked good from the very beginning so that's to be expected."
But he said it would be a mistake to draw any hard and fast conclusions from the evidence thrown up by testing.
"We'll have a better idea I guess this time next week where we stand but still then we won't know what fuel loads everybody is on and who is showing their cars and who isn't."
Hamilton, with a new two-year deal that will earn him £40 million ($52 million) a season, is buzzing after the winter recess.
"I feel in the best shape that I've been in," he assured his fans, as he embarks on his quest for a sixth championship and record sixth consecutive constructors' crown for Mercedes.
On a sixth title, he shrugged: "I am not even focusing on that. It's a new season, a new year, a new chapter and we have to approach like it's our first.
"We're going for number one, that's how I approach it.
"But it's not something that I allow my peers to talk about. It's not what we are here for to focus on that number."
He dismissed out of hand any notion of complacency setting in at his all-conquering team.
"We are always driven to do something new and to break boundaries and do stuff that no one else has done before but we are fully aware of the work that it will take to deliver something like that and it's by no means like 'yeah we are going for the sixth' like it's a relaxed thing.
"It's not like that."
Hamilton's bid for title number six begins at the Australian Grand Prix in Melbourne on March 17.
On the track Wednesday, Kvyat led the time sheets after 134 laps as Williams got their new car out of the garage after missing the first two days.
Deputy team principal Claire Williams said finally getting onto the circuit was "a big relief".
She added: "It’s embarrassing not bringing a race car to a circuit when everyone else has managed to do that, particularly a team like ours that has managed to bring a race car to testing for the past 40-odd years."
