Lewis Hamilton shrugged off Ferrari's testing pace on Wednesday while recognizing that he and Mercedes faced their toughest challenge yet as they seek to stretch their Formula One domination into a sixth successive season.

Ferrari, overall runner-up last year, has led the timesheets for the first two days of testing at Barcelona's Circuit de Catalunya and has also done plenty of laps with its new SF90 car.

Sebastian Vettel, the four-time champion who is partnered by young Monegasque Charles Leclerc this year, said on Monday his Ferrari had been "close to perfection".

Mercedes, winner of both titles for the past five years, has also been pounding out the mileage without setting any eye-catching times.

"They (Ferrari) have been looking great," Hamilton, who will be chasing his sixth title, told reporters. "For us it's been just digging deep, trying to understand the car. Pretty much the same as the beginning of every year.

"The Ferraris always look strong, particularly in the last few years they look very strong right at the beginning so it's to be expected."

Ferrari won the first two races of last year with a car that seemed quicker than the Mercedes but the champion came back strongly and Hamilton ended the year with 11 wins to Vettel's tally of five.

Last year Ferrari — which won six successive constructors' titles between 1999 and 2004 — was fastest in testing but it was still Hamilton who put his Mercedes on pole position at the first race in Melbourne with a circuit-record time.

"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.

"I just try to focus on our job. I don't know what everyone's doing, there's different fuel loads. I've been in this business a long time so I know how it goes over the first week and into the second.

"It's not a time where we need to be focusing on others. We let everyone else do their thing and really try to focus on digging deep, making sure our processes are better than ever before, analyzing the data better than ever before."

Hamilton said the W10 Mercedes felt similar to last year's W09, despite aerodynamic rule changes, and he was in the best shape of his life physically.

"I'm here for round seven with the team," he said. "This is going to be the most challenging year, I think, of our partnership.

"And I still love racing so nothing really changes there."