They might have ended the opening day of practice for this weekend's Spanish Grand Prix in a familiar position - ensconced one-two at the top of the timesheets - but Red Bull Racing duo Sebastian Vettel and Mark Webber remain wary indeed about the potential of McLaren-Mercedes around the Circuit de Catalunya.

McLaren has struggled in Barcelona in recent years as its designs have tended to excel more around high-downforce tracks and struggle more in high-speed corners, of which the Spanish circuit has no shortage - but prior to the 2010 edition, the team had hinted that the tables might just be set to turn [see separate story - click here].

The Woking-based outfit duly led the way in FP1 as Lewis Hamilton and current world championship leader Jenson Button paced the field, with Webber and Vettel respectively just fourth and fifth. In the afternoon session, however, the Milton Keynes-based squad hit back - setting the scene for a mouth-watering battle royale between the two teams in qualifying on Saturday.

"We've got a few changes on our car, but I don't think anything as significant as perhaps some of the other teams have," mused Vettel, F1's youngest-ever grand prix-winner. "There's a little bit everywhere on ours, and I think we've proved today that our changes work.

"I think the lap times are irrelevant from a Friday, though. It's good to be fast, but I think the McLarens are quick here - I don't think they were pushing hard this afternoon and I think it will be much closer tomorrow."

"That was a good day for us," corroborated New South Wales native Webber. "We had a smooth run and a nice FP1 and FP2, with no interruption from the weather or things like that. The guys worked faultlessly in the garage, the driver did a little bit as well and it was a good day.

"Don't read too much into the times; McLaren probably weren't showing everything today. It was a very late night for the team last night, but a positive day today and hopefully we're well-prepared for tomorrow."