Mercedes' Toto Wolff predicts more competition from 2015 rivals

Wolff (centre) said Rosberg (left) and Hamilton would drive alternate days at the opening four-day test in Jerez

World champions Mercedes say they expect a close fight from their rivals from the start of the new season.

Other teams fear it will be tough to catch up in 2015 after Mercedes dominated last year, winning 16 of the 19 races.

But team boss Toto Wolff said: "We are certain our rivals will be competitive from the opening race."

Mercedes will unveil their new car, the W06 Hybrid, on the first day of pre-season testing in Jerez on 1 February.

The season starts in Melbourne, Australia, on 15 March and runs for 20 races, finishing in Abu Dhabi on 29 November.

Mercedes said Nico Rosberg would drive the car on its first day of testing in Spain, with world champion Lewis Hamilton taking over on the second.

The two will alternate for the remainder of the four-day test.

Wolff added: "The workload this winter has been as great as ever. Where last year we were preparing for an all-new formula of racing, we are now faced with the big challenge of making the next step in our development.

"We have to focus on every detail if we want to build any advantage this year and that is what we have done."

Mercedes have become the third top team to announce when their car will be unveiled.

McLaren will reveal the first car of their new engine partnership with Honda on 29 January, with Ferrari following a day later on 30 January. Both launches will be online, via the teams' websites and social media outlets.

Jerez is the first of three pre-season tests, with succeeding ones at Barcelona's Circuit de Catalunya on 19-22 February and 26 February to 1 March.

Force India are unveiling their 2015 livery - but not their 2015 car - in an event in Mexico on Wednesday.