Highlights - European GP second practice
Two-time world champion Fernando Alonso set the fastest time in free practice for Sunday's European Grand Prix.
The Spaniard lapped the Valencia street circuit in one minute 39.283 seconds to top the times for Ferrari.
Red Bull pairing Sebastian Vettel and Mark Webber were close at hand in second and third positions, followed by Mercedes' Nico Rosberg.
Englishman Lewis Hamilton was fifth fastest in his McLaren-Mercedes, with team-mate Jenson Button taking ninth.
Ferrari's Felipe Massa caused the session to be stopped when he spun and stalled after 30 minutes and lost the following half an hour of running, but the Brazilian recovered to take seventh-fastest time.
Virgin driver Timo Glock was another in the wars. He lost the final 40 minutes after he stopped out on the circuit with a mechanical problem, while Renault rookie Vitaly Petrov had a spin before rejoining.
Red Bull drivers Vettel and Webber were first and second fastest on the harder 'prime' tyres before teams started fitting the softer 'option' tyres during the final half-hour of the session.
But Alonso, performing this weekend in front of his home crowd, looked extremely strong as he vaulted his Ferrari to the top of the times.
Much attention is focused on Ferrari, who have fitted to their car a new exhaust system that follows the trend set by pace-setters Red Bull.
This is estimated to improve the car's pace by at least 0.5secs a lap, and BBC F1 pit-lane reporter Ted Kravitz said that the indications were the new parts were working well.
Kravitz added that the teams believed that McLaren, Red Bull, Ferrari and Mercedes were all evenly matched in Valencia.
Renault and Mercedes have also brought their versions of the system to Valencia.
Ferrari, featuring new update kit, carried Alonso to fastest time
Mercedes driver Rosberg used the update to go fastest in this morning's opening practice session, followed by McLaren-Mercedes pairing Hamilton and Button.
While Rosberg looks strong, team-mate Michael Schumacher finished just outside the top 10 on his first visit to the Valencia street circuit.
Force India again look to be contenders for points, with Adrian Sutil taking the eighth spot and Vitantonio Liuzzi in 12th.
The top 10 in both sessions was completed by Rubens Barrichello's Williams, the team working on their version of the rear-wing 'F-duct', which increases straight-line speed.
Alonso could not say whether the updates had improved his Ferrari.
"The car was good today but it was also in Canada (the last race)," he said. "When you try to find out the difference with the upgrades on a different track, with a different level of grip and downforce, it's very difficult to compare.
"The potential is there: we can be competitive. Red Bull are very strong, McLaren are very strong - although that wasn't so obvious in second practice. Nico (Rosberg) and Michael (Schumacher) can fight - they have important upgrades.
"We'll have to wait until tomorrow to see if we're fast or not. I was P1 here last year on Friday, and then I was on P8 on Saturday..."
Red Bull were pleased with their day, during which Vettel and Webber evaluated the team's version of the 'F-duct'.
"We need to see if we keep the F-duct in the car, we need to confirm that, but all in all I think the pace was promising today considering this shouldn't be our strongest track," said Vettel. "We made a step forward with the whole car - it was a good day for us."
Rosberg said that the Mercedes updates had made the car more competitive. "We made some progress, especially with the new parts that have come," he said.
"There is still a bit of a problem with making the tyres work, especially on this track today, but we're definitely a step closer to the front. I think we were very quick today, especially on high fuel."
McLaren are anticipating a tough weekend. "The car feels pretty good - the only problem is there are lots of quick cars out there," said Button. "The Ferrari looks very very fast, Red Bulls as normal, but even the Renaults and Mercedes look competitive as well.
"That's because teams bring out upgrade packages at different points during the season, and we don't have an upgrade for here.
"It evens out over the season but we've just got to fight it out here and get the best out of what we've got.
"You can never tell on a Friday but we're not as quick as we might hope."
The team didn't tell me (that Hamilton was behind) so I must apologise. It was my fault
Team-mate Hamilton, who leads the world championship, added: "They are looking very very competitive - more so on the single lap than race pace I think. I think our race pace is closer, but the grip that they look like they have through the high-speed corners will make it difficult for us to touch them."
Hamilton, the 2008 world champion, also played down an on-track mix-up late in the session with seven-time champion Schumacher that almost resulted in a collision.
"It's always very difficult to find a gap out there, and I saw him ahead of me, but I thought he was on a lap," said Hamilton. "And then all of a sudden he slows down to get a gap to the guy in front.
"There's no hard feelings and I just missed him, so it was no problem."
Schumacher said: "The team didn't tell me (that Hamilton was behind) so I must apologise. It was my fault."
Two teams gave their reserve drivers a run-out in the opening session.
Scotsman Paul di Resta replaced Sutil at Force India. He was 0.057secs slower than Force India team-mate Liuzzi and took 16th place.
Hispania Racing's reserve, Christian Klien, was 0.220secs slower than Bruno Senna and finished 23rd. He was then replaced by race driver Karun Chandhok.
Senna's car prompted a session stoppage when it lost a mirror, which was run over by the Toro Rosso of Sebastien Buemi, prompting a red flag to clean up the debris.
Tomorrow's practice session starts at 1000 BST.
Highlights - European GP first practice