World champion Sebastian Vettel was in impressive form as he set the pace in European Grand Prix practice.
Driving a car with aerodynamic updates, the German was 0.131 seconds faster than Force India's Nico Hulkenberg.
Sauber's Kamui Kobayashi was third ahead of the Mercedes of Michael Schumacher and Williams's Bruno Senna.
Scot Paul di Resta was sixth for Force India ahead of Fernando Alonso's Ferrari. McLaren drivers Jenson Button and Lewis Hamilton were 12th and 14th.
The mixed-up field was a reflection of a session that was disrupted by a crash by HRT driver Pedro de la Rosa, which interrupted several drivers as they tried out one-lap runs on the softer of the two tyres.
The drivers did not seem to get as big an advantage from the softer of the two tyre options as expected, and the presence of Hulkenberg and Senna in the top five underlines the fact that these times should not necessarily be seen as an indication of competitive form.
"I would say it is a very open race - there is no clue who I would put as favourite at the moment," said BBC 5 live analyst Jaime Alguersuari.
The Red Bulls looked quick in both sessions as Vettel and team-mate Mark Webber ran new rear bodywork aimed at improving downforce at the rear of the car.
Vettel was second fastest in the morning session, ahead of Webber, who was ninth in the afternoon after complaining of problems with his downshift.
Vettel said: "We have some new bits but I think lap times are not allowing us to have a fair conclusion yet. The most important thing on Friday is to get a straight day, a good day of testing.
"It's pretty tricky here. The track gets used once a year, and this morning it was very dusty and very slippery, a lot of wind. This afternoon it was a lot more consistent. I don't know what other people did, but for us it was important to stick to the programme, which we succeeded at."
The Mercedes cars also looked fast - Rosberg was on course to do a time similar to Schumacher's but had to abort it his lap as a result of De la Rosa's crash.
The Lotus drivers Romain Grosjean and Kimi Raikkonen were only eighth and 11th fastest on headline lap times in the afternoon session. The first session was topped by Williams driver Pastor Maldonado, who took his maiden win at the Spanish Grand Prix last month.
Button was fourth in the morning, ahead of Alonso, with Di Resta sixth and Hamilton eighth.
Both sessions were highly competitive - in the first session just 0.292secs covered the top nine cars. In the afternoon, the top 10 were separated by just 0.6secs.
The tight field suggests another close, unpredictable race in a season that has produced seven different winners from the first seven races.