BARCELONA, Spain -- Formula One championship leader Sebastian Vettel posted the fastest time ahead of archrival Fernando Alonso in the second practice session of the Spanish Grand Prix on Friday.
Alonso had the best time in the rain-affected first session, where Vettel drove more cautiously, but the Spaniard finished .017 seconds behind the German as sunshine bathed the Circuit de Catalunya for most of the afternoon's run.
Vettel clocked 1 minute, 22.808 seconds -- considerably faster than the 1:25.252 Alonso managed for Ferrari in the morning run.
"Overall, we can be happy with today," Vettel said. "This afternoon was fairly conclusive, but this morning wasn't really much good with the weather."
Vettel's Red Bull teammate Mark Webber was third quickest and the Australian was .083 behind, with Finland's Kimi Raikkonen fourth fastest in his Lotus.
"We couldn't have got more out of it today really, the weather made things a bit tricky," Webber said. "We had a few things to test this morning which we had to move to the afternoon when the weather was better, but it was okay. We still have work to do tonight."
Saturday's qualifying promises to be highly competitive, with the top six in the second practice separated by .332 seconds. Ferrari's Felipe Massa was sixth, followed by Mercedes driver Lewis Hamilton.
"The team have done a great job to bring our upgrades here this weekend and whilst I could feel the impact immediately, it's difficult to quantify how much of an improvement has been made at this early stage," said Hamilton, who has two third-place finishes this season. "After the first session this morning, I felt there was some good promise and the car was well-balanced, but I was a little less happy in the different conditions this afternoon."
Every driver apart from Max Chilton and Paul di Resta managed at least 30 laps, with Nico Rosberg getting 45 laps out of his Mercedes.
Rosberg thinks the car needs more durability on a heavier fuel-load to go along with its excellent short-burst speed.
"It seems that we are quick again (for) one lap but we need to focus very much on achieving a good race pace," the German said. "There is still a lot work to do but in general I felt that we had a good test program today and learned quite a lot. It's difficult to manage the tires in these conditions."
Vettel, who finished the first practice down in 19th place, flew out as soon as the afternoon session began.
The track design, with its long corners and changes in speed, offers a good test for the car upgrades providing it stays dry, and the afternoon conditions were perfect. Times dropped considerably as drivers tested out Pirelli's new modified hard tires.
Pirelli has made them slightly harder and longer-lasting after complaints they were wearing out too quickly, leading drivers to save up tires for race day and cutting short their practice runs.
"Difficult to say too much (about) the new hard tire. We didn't expect any miracle and I don't think it's a massive difference," Vettel said. "We see how it behaves on Sunday and I don't think it helps us in terms of changing the race, so we will see a similar amount of pit stops as the last race (in Bahrain)."
But there appeared to be an issue with Di Resta's rear tires, however, when he went off track and banked onto the grass after completing 25 laps.
"We're going to investigate the cause of that fully," Pirelli motorsport director Paul Hembery said. "Whatever the cause this is not something we like to see, and once we have established all the facts we will decide whether some further modifications to the tires are required to help avoid this type of issue."
The upgrades are likely to prove especially crucial to McLaren, however, as Jenson Button and Sergio Perez try to kickstart their bitterly disappointing seasons.
Perez tried out the car's new nose late in the afternoon session, which he finished in 13th place, a place behind Button, who did not do a timed lap in the first practice.
"The thing is with this sport everyone's moving forward," Button said. "We're working flat out to improve at the moment."
Button and Perez have managed only one fifth-place finish between them, and have had to clear the air after clashing during last month's Bahrain GP.