Jenson Button whizzes around the Circuit de Catalunya during the second practice session
Jenson Button and McLaren laid down an early marker on Friday when the Englishman topped the times in opening practice for Sunday's Spanish Grand Prix as drivers sweated over tyres for the race.
Button, the 2009 world champion, overcame problems with under-steer to clock the fastest time in the afternoon second free practice session in one minute and 23.399 seconds.
This gave him a comfortable advantage of more than one tenth of a second over defending double champion Sebastian Vettel of Red Bull at the end of an intriguing open day run in warm sunshine at the Circuit de Catalunya.
Button made the most of Pirelli's unforgiving softer tyres, much criticised for their lack of durability by other drivers including seven-times champion Michael Schumacher, as McLaren delivered a strong showing on the opening day of the European section of the 2012 season.
"It was a positive afternoon. The car is running well, and our upgrades seem to be working. But there are still some improvements to be made," said Button.
"The Option tyre seems to be a bit easier for everyone as it's softer and therefore more receptive to generating temperature. In P2, we also started to find a way to get the Prime to work, but then, later in the session, we couldn't quite get back to that sweet-spot.
"If we can solve these issues with the Prime and get it into its correct working range, I think we can be quite competitive this weekend."
On a warm early Spring day, when the air temperature rose to 29 degrees Celsius and the track temperature to 43, Nico Rosberg, who won his maiden Grand Prix for Mercedes in Shanghai earlier this year, came third fastest ahead of 2008 champion Lewis Hamilton of McLaren.
"That was very different to winter testing here and a lot tougher on the tyres," said Rosberg.
"It has been a very challenging day first of all to see how to get the car right for qualifying and especially how to get it right for the race.
"I don't know where we are on tyre wear. We will have to look through the data now. Tyre management is going to be crucial here."
Asked about tyre degradation, Schumacher said: "That's a challenge. We have to find out more and analyse it to see what we did compared to the other guys. We have four of them (tyres) each time we go out and it's the same for everybody..."
Kimi Raikkonen, the 2007 champion, was fifth fastest for Lotus ahead of team-mate Romain Grosjean and seventh placed Mark Webber in the second Red Bull.
Schumacher, in the second Mercedes, was eighth while the two Ferrari men -- Felipe Massa and local hero Fernando Alonso -- were 11th and 14th respectively.
Afterwards, Webber also expressed concerns about the tyres in use and their influence on the event, hinting that he believed several top drivers would sit out the final top-ten shootout part of qualifying because it may be more important to preserve their tyres than fight for an improved grid position.
"It is always nice to finish qualifying with a few sets of tyres. You always want to try and get through all three sessions with minimal damage on your tyres, and there will be no exception this weekend - we will see," said the Australian.
"We could see people sit out Q3, those that just get into Q3, and they will do that to save tyres."
Vettel added his own veiled support for Webber's view.
"The most important thing is points -- and points you get on Sunday.
"Sure, I know the reason we go out on Saturday is to qualify as high as we can, ideally to get pole, so nothing has changed, but we have seen that during the race there is a lot of things happening with the tyres.
"Where you start on Sunday does not automatically mean that is the position you will finish."