Lewis Hamilton set a searing pace to head Mercedes team-mate Nico Rosberg in second practice at the Spanish GP.
Hamilton was 0.449 seconds quicker than Rosberg and nearly a second clear of Daniel Ricciardo's Red Bull in third.
Ferrari's Fernando Alonso was fourth, 1.6secs off the pace and 0.175secs ahead of team-mate Kimi Raikkonen.
Red Bull's Sebastian Vettel did not take part in the session after a wiring loom failed after four laps of the first session.
Team principal Christian Horner said the problem was unrelated to a chassis change the team made for Vettel ahead of this weekend.
The German said the car was swapped for one used in testing as a "sanity check" after the four-time world champion was out-paced by Ricciardo in the last two races.
Mercedes again appear to be in a league of their own, and Hamilton had a clear advantage over Rosberg both on single-lap qualifying pace and on what was an extremely impressive race-simulation run on heavy fuel in the final part of the session.
"It's going really well," said Hamilton. "It's been a great day. It feels like a long break and to get back in the car and feel the positive steps made with the car... Big thank you to the team because the car's feeling a lot better this weekend and today has been a smooth day.
"I have not had a Friday like this for a long time."
BBC F1 analyst Allan McNish said: "It's all about Mercedes, and Hamilton had an advantage on long-run and short-run pace. After that, Ricciardo looks in good shape and Raikkonen is pretty much a match for Alonso for one of the first times this season, so he is obviously much more in tune with the Ferrari here."
Red Bull look to have made a step to move them clear of Ferrari in the battle to be next best, which Alonso won in the last race in China.
The team have introduced some aerodynamic and mechanical upgrades, while engine partner Renault is said to be getting close to being able to deploy the maximum allowable amount of hybrid power at all times, a significant step forward as it attempts to make up its performance deficit to Mercedes.
However, Red Bull team principal Christian Horner said there was still plenty of work to be done before Renault were a match for Mercedes in the engine battle where the world champions are losing most, if not all, of their time.
"The integration of the engine, turbo and hybrid system is still quite crude," Horner said. "It's about getting it all working together in harmony. It's not just about when the driver is on the power, but also in harvesting."
He added: "I'm not worried about Ferrari. We're 0.5secs off a Mercedes at the moment."
Kevin Magnussen beat team-mate Jenson Button to sixth place by just 0.023secs as McLaren tried a number of upgrades in an attempt to improve their poor form. They appeared to have moved up the field but were still more than two seconds off the similarly-powered Mercedes cars.
Williams' Felipe Massa was eighth, ahead of the improved Lotus of Pastor Maldonado, Toro Rosso's Daniil Kvyat and Force India's Nico Hulkenberg.
Toro Rosso's Jean-Eric Vergne was 12th fastest, but was given with a 10 place grid penalty and his team ordered to pay £25,000 after a rear wheel came off his car shortly after leaving the pits for a tyre change.
Ricciardo was given a 10-place grid penalty for being sent on his way after a pit stop in the second race of the season in Malaysia with a loose wheel.