Sebastian Vettel sets second practice pace, as Hamilton retires

World champion Sebastian Vettel set an ominous pace in his Red Bull on the first day of practice at the season-opening Australian Grand Prix.

The world champion headed both practice sessions in a manner which suggested he will be very difficult to beat as he goes for a fourth straight world title.

Mercedes driver Nico Rosberg was third, with team-mate Lewis Hamilton seventh.

Lotus's Romain Grosjean and Kimi Raikkonen were fourth and fifth ahead of Ferrari's Fernando Alonso.

Mercedes' pace suggested that they have indeed made a step forward towards the front this season, as pre-season testing had suggested.

But their session ended with both cars off the track.

Rosberg stopped with a gearbox problem while Hamilton slid off the track. Team boss Toto Wolff said there had been a problem with the 'bib', the front part of the floor of the car, which had caused him to lose grip and go off.

Hamilton was the faster of the two in the first session by half a second but had a messy second session.

Hamilton looked like he might have been able to challenge Vettel for ultimate pace - he had set a faster time in the first sector of the lap than Vettel before making a mistake at Turn Nine and having to abort it.

Mercedes appear to be competing with Lotus and Ferrari to be the fastest team behind Red Bull, who looked very impressive throughout the day.

Vettel's fastest time was set in conditions far less than optimum. It was on his second flying lap on the super-soft tyres, when they would have lost the peak of their grip, and he said he was having problems with his Kers power-boost system.

That suggests Vettel could have gone at least 0.5 seconds quicker than he did - a margin that was borne out by his superiority over the field on the medium tyre earlier in the session.

The Red Bull also set an impressive pace on used medium tyres - Vettel was only 0.5secs slower after 10 laps than he had been on his fastest lap, suggesting that tyre degradation will be nowhere near as bad as it has been in pre-season testing.

"Overall it was a good day for us, pretty seamless, no trouble with the car. Good fun to be out there," said the world champion.

"For sure it feels good to know you are kind of up there somewhere. There is room for improvement but we need to keep our eyes open and see what happens."

Vettel looked in good shape during the longer runs later in the session when the teams try to simulate race conditions.

Vettel also headed the first session, by 0.078secs from Ferrari's Felipe Massa, who was eighth in the second session.

The day's running, inconclusive though it was, suggested that four teams will be fighting it out at the front initially - but Ferrari, Mercedes and Lotus look like they may be fighting to be Red Bull's closest rival.

Pre-season testing had suggested McLaren were struggling for pace and Friday in Melbourne's Albert Park seemed to confirm that.

Their sporting director Sam Michael admitted: "We are not where we need to be at this point in time.

"We made quite a few changes to the car over the winter, and we still believe they will be good for the course of the season.

"But we are still unoptimised in the areas we need to be, which comes from our own knowledge of where they are not as good as what they should be.

"So we still have some work to do on the car. But it's a long season."

The forecast for qualifying on Saturday is for rain, which could provide a mixed-up grid ahead of an expected dry race on Sunday.

Second Practice top ten

1. Sebastian Vettel - Red Bull - 1:25.908

2. Mark Webber - Red Bull - 1:26.172

3. Nico Rosberg - Mercedes - 1:26.322

4. Kimi Raikkonen - Lotus - 1:26.361

5. Romain Grosjean - Lotus - 1:26.680

6. Fernando Alonso - Ferrari - 1:26.748

7. Lewis Hamilton - Mercedes - 1:26.772

8. Felipe Massa - Ferrari - 1:26.855

9. Adrian Sutil - Force India - 1:27.435

10. Nico Hulkenberg - Sauber - 1:28.187