Defending Formula One world champion Lewis Hamilton said that bitter rivals Ferrari looked to have developed by far the fastest car, potentially turning the season ahead into one of the most challenging, as pre-season testing at the Montmeló race track near Barcelona drew to a close.
The blood-red Prancing Horse team from Italy also began as the fastest team last season, but due to a combination of errors Hamilton's Mercedes squad leapfrogged the Italians and he became only the third driver ever to claim five world championships, following in the tracks of legendary Argentine racer Juan Manuel Fangio and Germany's Michael Schumacher who went on to claim an unprecedented eight titles before retiring.
"Last year, (Ferrari) arrived here with a car that was working well, but they've done even better this year," Hamilton said on Friday.
"This is going to be the toughest battle yet, which is exciting. Ferrari, their pace is very, very good at the moment, so the challenge is going to be harder than ever," he added.
Just three thousandths between Vettel and Hamilton...
Hamilton said that as a consequence, Mercedes would have a hill to climb to be able to match the speed and handling of Ferrari, whose cars have dominated the pace at pre-season testing over the last three days in Spain's Circuit de Barcelona-Catalunya.
Hamilton's teammate, Valtteri Bottas, pushed hard on the softest tyre compound, Pirelli's C5, which is capable of giving the fastest speeds, but at one minute 16.561 seconds, he was a third of a second slower than the Ferraris on the same rubber.
Still, Bottas was able to wring the second-fastest time, behind Ferrari's four-time world champion Sebastian Vettel.
Nevertheless, the Finnish driver said there were many things to improve as he emerged from his Mercedes after his fast lap.
Hamilton said that the gap in pace with Ferrari could be even greater.
"I think it's potentially half a second, something like that," Hamilton said.
"But we will be analyzing a lot from this test and there will be some mods that we'll try and implement before (Australia). There's obviously not a lot of time, but over this next week, hopefully, we'll gain another tenth at least just in our understanding of the car."
Vettel pushed his Ferrari hard on Friday and topped the timings after his new teammate Charles Leclerc had done the same the previous day.
The only concern was that Leclerc's car ground to a halt on the track due to a problem with its exhaust system.
Vettel had already raised concerns by crashing his car on the first day of testing on Wednesday and having to be taken to the track's medical facility for a checkup given the violence of his impact against the crash barriers.
Daniil Kvyat showed that Toro Rosso was adapting well to its new Honda power unit by going third fastest.
The Russian managed to clock faster times than his former teammate Carlos Sainz who has made the move to McLaren.
Kimi Raikkonen was an early casualty on the track when his new Alfa Romeo C38 ground to a halt, causing the organizers to red-flag the first practice session of the day.
Hamilton said that while it was clear Ferrari was setting the pace, it would take some time after the first race of the season, from March 14-17 in Australia, to know how hard the challenge was likely to be.
He insisted that he had confidence in himself and in his team to be able to fight hard to catch Ferrari.
"Melbourne will be kind of the first time you get a sight of it, and then, as always, every year I say it's a good four races before you fully know just where you stand," he said.
"I have the confidence that I have the team to do the job," Hamilton said.
"We're the most united that I think we've ever been."