Lewis Hamilton showed he is not ready to give up his world title without a fight after Sebastian Vettel produced the fastest lap ever seen in winter testing at the Circuit de Catalunya in Barcelona.

Vettel's time of one minute and 16.221 seconds was quick enough to leave him top of the timesheets and earn him a slice of Formula One history.

But the 2019 world championship battle between Ferrari and Mercedes looks set to be as fierce and unrelenting as it was last year after Hamilton finished within three-hundredths of a second of his German rival.

Before taking over his Mercedes W10 for the day from team-mate Valtteri Bottas, Hamilton used his lunchtime media session to suggest that Ferrari had a half a second advantage over Mercedes.

But that did not appear to be the case as Hamilton just failed to pip Vettel's time during the final session.

"I think the gap (to Ferrari) is potentially half a second," five-time world champion Hamilton had earlier told reporters.

"Ferrari's pace is very, very good at the moment. The challenge is going to be harder than ever."

Bottas had set a time of one minute 16.561 to trail Ferrari by around three-tenths of a second.

Vettel, returning to the cockpit for the first time since his crash on Wednesday, had eclipsed Ferrari team-mate Charles Leclerc's time by one-hundredth of a second.

But Vettel had his own difficulties on the final day of testing when an electrical problem halted him on the track early in the afternoon session, bringing out the red flags.

With the team claiming the issue would take too long to fix, the car was put back in its box and Ferrari ended their 2019 pre-season test early.

Hamilton, however, had no hesitation saluting Ferrari's winter work.

"Last year, (Ferrari) arrived here with a car that was working well," said the British driver.

"But they've done even better this year. That's OK – we don't mind the challenge. We'll fight, it just means we've got to work harder.

"We will be analysing 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.

"Everyone also has different power modes and all those kind of things, and fuel usage – you don't know what fuel everyone else is on.

"So 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.

"I have the confidence that I have the team to do the job. We're the most united that I think we've ever been."

The eight days of testing came to an end with five drivers hitting the 100-lap mark.

Aside from Vettel, who completed 110 laps, Toro Rosso's Daniil Kvyat, McLaren's Carlos Sainz, Alfa Romeo's Kimi Raikkonen and Racing Point's Sergio Perez all reached three figures.

Behind Bottas was the Renault of Nico Hulkenberg, who had a time of one minute 18.843, with Kvyat half a second back.

Max Verstappen managed only 29 laps before a gearbox issue sidelined the Red Bull driver.

The opening round of the 2019 world championship is scheduled to start in Melbourne on March 17.