MELBOURNE (Reuters) - World champion Sebastian Vettel put his rivals firmly in their place by dominating both practice sessions on the opening day of the Formula One season at the Australian Grand Prix on Friday.

Red Bull Formula One driver Sebastian Vettel (L) of Germany smiles with Ferrari Formula One driver Fernando Alonso of Spain during the official news conference at Albert Park circuit in Melbourne March 14, 2013, ahead of the Australian F1 Grand Prix. REUTERS/Brandon Malone

The German, embarking on his quest for a fourth successive world title, steered the car he has nicknamed 'Hungry Heidi' around the Melbourne street circuit in one minute 25.908 seconds half an hour before the end of the second session.

That lap topped the time sheets for the day, edging his Australian team mate Mark Webber (1.26.172) into second place as the Red Bull team scattered a few seeds of doubt over the consensus that the gap to the rest of the grid was narrowing.

"Overall it was a good day for us, it was pretty seamless and there was no trouble with the car," the 25-year-old Vettel told reporters.

"After all the mileage of testing, it was fun to be out there - this circuit doesn't get easier, it's a good challenge."

Nico Rosberg (1.26.322) was the best of the rest on what would have been an encouraging day for Mercedes had he not later ground to a halt with a gearbox problem moments after team mate Lewis Hamilton had driven over the gravel and into a wall.

Former world champion Kimi Raikkonen, third in the championship last year, and Romain Grosjean grabbed the fourth and fifth best times to confirm some impressive showings for Lotus in pre-season testing.

Twice world champion Fernando Alonso and his team mate Felipe Massa had more competitive first than second sessions for Ferrari, finishing sixth and eighth fastest overall to sandwich the unfortunate Hamilton.

"We already knew we were not the quickest and that was confirmed today," said Spaniard Alonso, runner-up in the championship last year.

"The weather is expected to change completely from tomorrow, with wind, rain and a significant drop in temperature, which could influence Sunday's race."

LONELY WALK

Hamilton had started brightly on a warm and sunny day at Albert Park and finished fourth quickest in his first practice session for Mercedes after his switch from McLaren.

The 2008 world champion faced the lonely walk back to the paddock seven minutes from the end of the second session, however, after a crash the team said had been caused by a bodywork problem which caused him to understeer.

"Overall it was a good day, if perhaps not the smoothest," said the Briton. "I'm happy with where we are and to see Nico in third place on the timesheets shows that we're pretty competitive.

"The Red Bull is massively quick as it always is," he added. "They've obviously been sand-bagging it the whole time in winter as expected and all of a sudden they've pulled out (that) time."

German Adrian Sutil made an excellent start on his return to Force India after a year out of the sport by setting the ninth quickest time overall, outperforming not only his team mate Paul Di Resta but also both McLarens.

Jenson Button, who has won three of the last four Australian Grands Prix, managed only the 11th fastest time - 2.386 seconds off the pace - with his new team mate, Mexican Sergio Perez, lapping 13th quickest.

"It was one of the most difficult days I can remember," said team principal Martin Whitmarsh.

"We've made a lot of changes to the car for this season and we don't fully understand how to get the best out of it yet."

None of the five rookie drivers set the world alight but Dutchman Giedo van der Garde had the worst of it, banking his Caterham on the trackside gravel after miscalculating the same turn six that later ended Hamilton's day.

A third practice session takes place on Saturday before qualifying for Sunday's race.

(Editing by John O'Brien and Clare Fallon)

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