MELBOURNE, Australia - World champion Sebastian Vettel put his rivals firmly in their place by dominating both practice sessions ahead of the season-opening Australian GP.Vettel, embarking on his quest for a fourth consecutive World title, steered the car he has nicknamed "Hungry Heidi" around the Melbourne street circuit in 1min25.908 in the second session.That lap topped the time sheets for the day, edging his team mate Mark Webber (1min26.172) into second place as Red Bull team scattered a few seeds of doubt over the consensus that the gap to the rest of the grid was narrowing.'IT'S A GOOD CHALLENGE'Vettel said: "Overall it was a good day for us, it was pretty seamless and there was no trouble with the car. 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 (1min26.322) 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 in 2013, and Romain Grosjean grabbed the fourth and fifth best times to confirm some impressive showings for Lotus in pre-season testing.Double 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.Alonso said: "We already knew we were not the quickest and that was confirmed today. 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 FOR LEWISHamilton finished fourth quickest in his first practice session for Mercedes after his switch from McLaren. The 2008 World champion had a 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.Hamilton said: "Overall it was a good day, if perhaps not the smoothest. 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. 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."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.DIFFICULT DAY FOR McLARENJenson Button, who has won three of the last four Australian races, managed only 11th fastest, 2.386sec off the pace, with his new team mate, Sergio Perez, lapping 13th quickest. Team principal Martin Whitmarsh said: "It was one of the most difficult days I can remember. 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 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 March 15 before qualifying for the race on March 17.