Reigning F1 world champion Lewis Hamilton may not have been behind the wheel of the new Mercedes W06 on day one of testing at Jerez, but he already expects the coming season to be a tough one.

Although he didn't watch much of the day's action on track, the Briton was impressed with team-mate Nico Rosberg's 156-lap haul, but conceded that it would be easier for the rest of the field to make bigger gains with their 2015 cars than Mercedes possibly could.

"I think last year was an exceptional year, and to be able to match that will be very tough," he acknowledged, reflecting on a 16-win season for the eventual champions, "We go into the second year of this car and engine and, for the others that were further behind, it should be easier to make steps forward as our car was pretty amazing last year so to make steps will be harder and they'll be smaller.

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"It's like going up a mountain... at the beginning, it is quite easy to make steps forward but it gets steeper and steeper and steeper, and we are closer to the top of the mountain and others were not. But I think the guys have done a great job to improve the car even though we were so good last year - it's easy to make it worse, hard to make it better."

Although the differences between them last year were largely laid to rest after Abu Dhabi, Hamilton admits that he wants nothing more than to win a third world championship, and that naturally means beating Rosberg into at least second place - even in testing.

"It's been an amazing start for him today - and it means I've got to try and do 180 [laps] tomorrow!" he joked.