McLaren's Lewis Hamilton sprung a surprise by running Red Bull close for the fastest time in practice for the Spanish Grand Prix.
Hamilton split the Red Bulls, with Mark Webber setting the pace just 0.039 seconds faster than the McLaren.
It was an encouraging sign for McLaren, who have brought a number of new parts to Barcelona to improve performance.
Hamilton's team-mate Jenson Button was fourth, ahead of the Ferrari of Fernando Alonso.
All the teams have brought new parts to this race as they seek to enhance their competitiveness, and the first indications are that McLaren may have made the most progress.
"We made some small steps forward today," said Hamilton. "We improved a couple of small areas. The front wing seems to be a little bit better. Globally it seems to be not too bad."
When asked if his performance means McLaren are now closer to Red Bull, Hamilton replied: "I don't know. From the times it looks great. Usually when we get to qualifying they end up switching something on and they're half a second quicker. Undoubtedly they are still the quickest car."
The Red Bulls were quick to follow the lead of Renault's Vitaly Petrov by running on the softer 'option' tyres that will be used in qualifying, which Vettel has dominated so far this year.
Vettel set a lap that was two seconds faster than the quickest car up to that point, and when Webber then went faster still it seemed to prove the assumption that Red Bull would dominate this event.
"The weekend is off to a not bad start," said Webber. "The usual suspects are going to be there - Ferrari and McLaren. It's been like that for two and a half years so I don't think it's going to change here."
The Circuit de Catalunya is probably the toughest test of a car's aerodynamic capability on the calendar, an area in which Red Bull are known to excel.
Webber was on pole position by nearly a second here last year and went on to dominate the race and the team were expected to continue in that vein this season, in which their car has had a bigger general advantage than in 2010.
And practice suggested that qualifying and Sunday's race may be closer than expected.
After the Red Bull drivers had set their initial times, Button came out and could manage only to get within 0.7secs of Webber.
That moved Lotus reserve driver Karun Chandhok, acting as BBC 5 live's analyst, to say: "Red Bull look like they've got the best part of a second on the field. McLaren are 0.7secs away, which is about where we'd expect them to be."
However, Hamilton reset perceptions with his lap and Alonso also showed good pace at various times - he was held up by Renault's Nick Heidfeld on his quickest lap.
Alonso's team-mate Felipe Massa was 0.7secs adrift of the Spaniard and also had a narrow escape from the barriers after running wide at Turn Five.
"That was close, very close," Massa said over his car-to-pit radio. "Yes, I saw," replied his engineer Rob Smedley.
Red Bull appeared to have a trouble-free afternoon but they will be concerned about an apparent recurrence of problems with their Kers energy recovery and power-boost system.
This has been Red Bull's Achilles' heel so far this season - only in one race have the drivers both been able to use it without any problems. And in the first session, Vettel was at one stage told by his engineer Guillaume Rocquelin to stop using the system.
McLaren spent the day running through a series of aerodynamic changes to the car.
Hamilton and Button actually have two races' worth of updates here after they were forced to abandon plans to use new parts in Turkey two weeks ago because of reliability concerns.
However, Button revealed: "I'm not particularly happy with the balance of the car at the moment. I'm struggling with the rear end. It's pretty unpredictable. It's not something I like. It's something I've got to work on and improve for tomorrow.
"If I look at what Lewis is doing, I just don't have the balance [to do that time]. I'm a long way off at the moment. There is more pace in the car, we just have to unleash it."
Like McLaren, Ferrari had a new front wing, as well as a modified rear wing and, like Red Bull, a new floor.
The rear wing, which features a scoop out of the centre of the upper element, has attracted attention from governing body the FIA.
Race director Charlie Whiting described it as a "clever" interpretation of the rules, but added that he would decide overnight whether it was acceptable.
Among the teams for whom a lot rests on this weekend are Williams, who are hoping to make amends for the worst start to a season in their history by scoring their first points.
Rubens Barrichello was an encouraging ninth in first practice but only 14th in the second session.
His team-mate, the Venezuelan novice Pastor Maldonado had another difficult day after crashing in the first session at Turn 13.
Maldonado lost control through the corner, sliding wide onto the slippery outside of the track, where he was unable to stop the car continuing into the gravel trap and barrier.
BBC Radio 5 live analyst Maurice Hamilton said: "Once Maldonado got off line there was just no grip whatsoever. He lost it on the dirty side of the track. That's very embarrassing for him and very annoying. Maldonado needed a good clean weekend."
Williams have a raft of new parts here, including new wings and an exhaust-blown diffuser.
A curve ball was thrown into the teams' preparations for qualifying and the race by a new tyre that will be used in the grand prix for the first time this weekend.
Pirelli has brought a harder version of its 'hard' tyre to cope with the abrasiveness of this track. The aim was for it to last about five laps longer than the older version of that tyre. Both that and the 'soft' tyre have to be used in the race.
But after using the new tyre in practice the drivers felt it was a step backwards.
Hamilton described it as "a disaster", adding: "I don't know why they've brought that tyre because the previous one was pretty good.
"I think we were about 2.5secs off the pace with that tyre. Then we switched to the soft and it was fantastic, so I think you'll see most people driving on that tyre for the weekend.
"It looks like another three-or-four-stop race using as short a stint as possible on the new tyre."
Alonso said the lack of pace in the new tyre would cause problems in qualifying. The teams try to use as few sets of tyres as possible before the race because sets are limited and it is better to have more unused tyres for the grand prix itself.
So the preferred strategy for the drivers in the faster cars is not to use the harder tyre in the first part of qualifying, when the slowest seven cars are knocked out and left in the final places on the grid.
But Alonso said: "It's now difficult to think about using the hard tyre in Q1. Ninety per cent of the teams will use the soft tyre then."
Button agreed with his rivals' assessment of the tyre, but added: "We've all got to work with the same tyre so we've got to make the best out of it. You'd not want to do more than one stint on the tyre. It's something else to add to the mix."