Lewis Hamilton set the pace in Friday's first practice in Spain while Sebastian Vettel and Nico Rosberg were both hit with problems.
On a morning when all 11 teams were testing updates for their cars, it didn't take long for the drivers to get down to business.
Daniil Kvyat set the early benchmark time, a 1:37.986, before lowering that to a 1:36 and then heading back into the pits.
After a few minutes of silence, the drivers headed back out, putting their extra set of hard Pirelli tyres to good use.
Pastor Maldonado had a brief moment at the top of the timesheets before Fernando Alonso came through with a 1:29.874. Kevin Magnussen was second ahead of Jenson Button.
However, when the two Mercedes drivers ventured out it was all change. Lewis Hamilton hit the front with a 1:28.028 while Nico Rosberg went second quickest, a tenth behind.
Meanwhile there were problems at Red Bull Racing as Sebastian Vettel came to a stop out on track at the exit to Turn 5. He had completed just four laps at the time and was down in 15th place.
Team boss Christian Horner revealed it was a "full electrical shut down" that forced Vettel to stop.
Back in the action, Jenson Button and Daniel Ricciardo took control as the drivers headed out for a second stint. Button led with a 1:27.891 while Ricciardo was a tenth behind. As for Mercedes, the team confirmed an ERS overheating issue for Rosberg that would keep him off the track for the rest of the morning.
His team-mate Hamilton, though, ventured out with 20 minutes remaining and although considerably quickest through the second sector backed off.
The Brit finally completed a full lap with just a few minutes remaining and did so in emphatic style, posting a 1:27.109 to beat Button by 0.782s. He then went even quicker with a 1:27.023, extending his advantage to 0.868s on Button. Ricciardo finished Practice One in third place.
01 Lewis Hamilton Mercedes 1:27.023 17 laps
02 Jenson Button McLaren 1:27.891 +0.868 26 laps
03 Daniel Ricciardo Red Bull 1:27.973 +0.950 21 laps
04 Fernando Alonso Ferrari 1:28.128 +1.105 23 laps
05 Nico Rosberg Mercedes 1:28.168 +1.145 9 laps
06 Kimi Raikkonen Ferrari 1:28.337 +1.314 19 laps
07 Kevin Magnussen McLaren 1:28.423 +1.400 27 laps
08 Pastor Maldonado Lotus 1:28.744 +1.721 34 laps
09 Sergio Perez Force India 1:28.779 +1.756 18 laps
10 Felipe Massa Williams 1:28.791 +1.768 13 laps
11 Daniil Kvyat Toro Rosso 1:28.792 +1.769 24 laps
12 Nico Hulkenberg Force India 1:28.828 +1.805 17 laps
13 Jean-Eric Vergne Toro Rosso 1:28.859 +1.836 24 laps
14 Felipe Nasr Williams 1:29.272 +2.249 15 laps
15 Adrian Sutil Sauber 1:29.688 +2.665 16 laps
16 Jules Bianchi Marussia 1:29.820 +2.797 22 laps
17 Romain Grosjean Lotus 1:29.944 +2.921 21 laps
18 Giedo van der Garde Sauber 1:30.440 +3.417 22 laps
19 Max Chilton Marussia 1:30.748 +3.725 19 laps
20 Sebastian Vettel Red Bull 1:30.942 +3.919 4 laps
21 Kamui Kobayashi Caterham 1:30.997 +3.974 22 laps
22 Marcus Ericsson Caterham 1:31.421 +4.398 22 laps