MELBOURNE (Reuters) - Mercedes dominated free practice at the season-opening Australian Grand Prix on Friday, with Nico Rosberg edging his Formula One world champion team-mate Lewis Hamilton to top the timesheets in both sessions.

Rosberg, last year's championship runner-up, set the fastest lap of one minute, 27.697 seconds in the second session at Albert Park, topping Hamilton by one-tenth of a second.

Rosberg was also quickest in the first session, his best time of one minute, 29.557 seconds edging Hamilton as the constructors' champions carried their ominous form from winter testing onto the track.

Four-time world champion Sebastian Vettel was third fastest on his debut for Ferrari in the second session, with team-mate Kimi Raikkonen fourth best.

But with Vettel's fastest time over seven-tenths of a second adrift of Rosberg, the opening day's practice will strengthen belief that the first rounds of Formula One may be Mercedes versus the rest.