Triple Formula One world champion Lewis Hamilton hailed his "fantastic" Mercedes after wrapping up the first preseason test with another 99 laps ­before lunchtime on Thursday.

"It's been an incredible week of testing for us because we've done more mileage than we ever had before," the Briton told reporters at the Circuit de Catalunya before handing over to German teammate Nico Rosberg for the ­afternoon session.

"Before we came here the team were talking about doing 800 kilometers a day and I was thinking they are crazy. I've never done that in the past ... I've never had a week like this when the car has just kept going on and on and on."

Hamilton did 342 laps in total, 156 of them on the first day when the car was still fresh out of the garage. That works out at 1,592 kilometers, more than five race distances at the Spanish Grand Prix circuit.

With the V6 turbo hybrid power units now in their third year, reliability has been much better across the field than it was a year ago with far fewer red flags to stop the track action.

Even so, champions Mercedes have far outstripped their rivals in mileage terms, a better benchmark at this stage than outright pace because cars can have different fuel loads.

"The car looks fantastic," said Hamilton. "Obviously we don't know the pace of the car just yet but what we can say is that it's gone the distance and that's half the battle."

McLaren's Fernando Alonso, a ­double champion who had a nightmare 2015 due to the underperforming Honda ­engine, had his spirits lifted on Wednesday when he managed 119 laps.

They may have dropped again on Thursday, with the Spaniard managing only three laps in the morning and ­unable to set an official time due to a water leak.

"I think they are very, very strong," he said of Mercedes. "They are stronger than ever probably."

Ferrari, closest rivals to Mercedes last year, have looked quick with Sebastian Vettel fastest in the first two days and Finnish team mate Kimi Raikkonen sounding happy enough on Wednesday.

"I am sure it's going to be a better car than last year's," he said. "We are happy so far even if we know there are still a lot of things to do."