Vettel's 2010 triumph made him the youngest world champion in F1 history
By Sarah Holt
BBC Sport in Barcelona
World champion Sebastian Vettel put Red Bull on top on day two of the final pre-season test in Barcelona.
Vettel set the standard with the fastest time seen so far in testing at the Circuit de Catalunya with a lap of one minute 21.865 seconds.
McLaren's Lewis Hamilton was 1.023 secs back in fourth after reliability issues confined him to spells in the garage.
Felipe Massa was 1.4 secs behind Vettel - just ahead of Paul di Resta's Force India, as Ferrari rejoined testing.
Vettel refused to get carried away with Red Bull's pre-season pace and performance, and believes that only a race weekend will reveal if the team can carry forward the momentum from winning both titles in 2010.
The season begins with the Australian Grand Prix on 27 March after the Bahrain Grand Prix, originally scheduled for 13 March, was called off because of civil unrest.
"We will find out in Melbourne where we really are," said Vettel.
"I'm smart enough to know we all have zero points starting this year. We know what we have done last year but in a way it doesn't really help us this year. It's a new year and a new challenge.
"[The season] hasn't started, but there are so many new things that it's difficult to read how quick you are.
"But in general it looks quite good and the car is reliable. We have no big problems and, compared to the rest of the field, then I think we should be there or thereabouts.
"We need to wait one race to see exactly how good we are. Melbourne is not a real race track so it could be Malaysia or later when we see what is going on."
Jenson Button conceded that it would be
difficult for McLaren to challenge 2011's perceived pacesetters Red Bull and Ferrari
And McLaren's reliability problems returned to haunt them in Barcelona as Hamilton was halted by a recurrence of the hydraulics problem that affected Button at the last test and then an exhaust failure before lunch.
Things hardly improved for Hamilton when a further exhaust issue confined him to another prolonged spell in the garage in the afternoon before he returned to the track to set his best time of 1:22.888.
"This year has not been the best of years for our testing programme but we've learnt a lot about the issues," Hamilton told BBC Radio 5 live.
"We didn't have great mileage today but on the positive side the car felt quite a bit better to drive than at the last test here.
"We might turn up in Melbourne and find the car goes even better than it does now. We are a team that are able to close a gap but time will tell."
Red Bull and Ferrari ominously churned out over 100 laps each while Toro Rosso, Renault, Force India and Mercedes also clocked more than a century of tours.
Ferrari unveiled an updated aerodynamic package, which included a new exhaust layout, as they began their four-day programme at the final test.
"Ferrari started the testing season with 2010-style exhausts which exited either side of the gearbox," explained BBC pit-lane reporter Ted Kravitz.
"But Red Bull have designed the whole of their car to achieve clear air-flow off the top of the diffuser, and this meant enclosing the exhausts in a flat, square sleeve that runs along the top of the floor and exits just under the rear wing.
"Ferrari's design is very similar, but they are introducing it at the final test rather than the first one."
Mercedes also ran a modified exhaust, which exits in the middle of the outer body of their sidepods, in Barcelona.
Last week, team principal
Ross Brawn told the BBC that Mercedes needed to find an extra second in pace.
Nico Rosberg was 3.942 secs behind Vettel's best time but the German said he was 'hopeful' the team have taken a step forward.
"We had a few bits and pieces of the upgrade but obviously it doesn't fit together with the rest of the car which is still old," said Rosberg.
"There were glimpses of the performance gain that seemed very impressive which makes me very hopeful."
Scot Di Resta was also pleased as he took another step along the road as an F1 debutant as he completed 118 laps for Force India.
"I've done pit stops before in practice last year but trying them in race conditions was a new experience," said Di Resta. "It was a reasonable day for us."
There were still some teething problems for the rest of the field of F1 cars.
Pastor Maldonado marked his 26th birthday by prompting the first red flag of the day when his Williams ground to a halt because of a problem with the Kers device.
Di Resta caused another flag to unfurl with a fuel pressure problem before Jerome D'Ambrosio stopped his Virgin out on track because of an electrical issue.
Buemi also stopped later in the day because of a mechanical problem.
Toro Rosso continued to show signs of improvement, however, as the Swiss driver finished just half a second behind leader Vettel as well as adding useful mileage.
"Apart from the race simulation, I was also able to do a qualifying simulation," explained Buemi.
Hispania will roll out their 2011 car for the first time on Friday as the Spanish team also confirmed
Vitantonio Liuzzi as their second driver.
Day two results, fourth pre-season test at the Circuit de Catalunya:
1. Sebastian Vettel (Ger) Red Bull-Renault 1:21.865
2. Sebastien Buemi (Swi) Toro Rosso-Ferrari 1:22.396
3. Vitaly Petrov (Rus) Renault 1:22.670
4. Lewis Hamilton (GB) McLaren-Mercedes 1:22.888
5. Felipe Massa (Brz) Ferrari 1:23.324
6. Paul di Resta (GB) Force India-Mercedes 1:24.334
7. Kamui Kobayashi (Jpn) Sauber-Ferrari 1:24.436
8. Nico Rosberg (Ger) Mercedes 1:25.807
9. Jarno Trulli (Ita) Lotus-Renault 1:26.090
10. Pastor Maldonado (Ven) Williams-Cosworth 1:26.989
11. Jerome D'Ambrosio (Bel) Virgin-Cosworth 1:28.982