Sebastian Vettel lapped fastest for Ferrari on the penultimate day of Formula One's pre-season testing on Thursday but refused to accept the mantle of favourite from Mercedes rival Lewis Hamilton.

The four-times champion wrapped up his preparations for the opening Australian Grand Prix on March 26 with the fastest lap yet around Barcelona's Circuit de Catalunya -- a one minute 19.024 effort.

The German also completed a mammoth stint of 156 laps, equivalent to two and a half race distances in the space of a day.

Triple champion Hamilton was second quickest in 1:19.352, 0.328 slower, with both men lapping on the fastest ultrasoft tyres.

The Briton, who did 52 laps but shared duties with Finnish team mate Valtteri Bottas, had said on Tuesday that "Ferrari must be the favourite" and he saw nothing to change his mind two days later.

"I think Ferrari are bluffing and that they are a lot quicker than they are showing," said the Mercedes driver, who won 10 races last year but still lost out in the title battle to now-retired team mate Nico Rosberg.

"They're very close to us. It's difficult right now to say exactly who is quicker. But they are very close, if not faster."

Bottas said Mercedes were not relaxing and "not in a comfortable situation at all.

"Ferrari are looking very strong, Red Bull can always surprise, and other teams can be strong. But Ferrari look solid, strong and fast -- and who knows what people will bring to Melbourne," added the Finn.

Ferrari, the sport's glamour team who are still hurting after a 2016 season without a win that left them third overall, have gone about testing with minimum fanfare and little in the way of media activity.

With plenty of pressure on them, they have also tried to play down expectations -- which Vettel continued to do on Thursday.

"I think if you look at the amount of laps Mercedes has done, if you look historically how slow they were going in the test, how much they were able to ramp it up for the races...I think they're the ones to beat," he said.

McLaren, struggling to get their Honda engine up to speed, had another difficult day with Belgian rookie Stoffel Vandoorne seventh fastest but doing only 48 laps and twice bringing out the red flags in the morning session.