Lewis Hamilton headed team-mate Nico Rosberg as Mercedes were in a league of their own in Friday practice at the Bahrain Grand Prix.
Hamilton, winner in Malaysia last weekend, was 0.365secs up on the German in the night-time second session.
Mercedes were a second clear of the field. Ferrari's Fernando Alonso was third from Red Bull's Daniel Ricciardo.
Felipe Massa's Williams was fifth from McLaren's Jenson Button and Red Bull's Sebastian Vettel.
"It is good to come off a positive weekend and come here with the momentum," said Hamilton. "We still have a lot of work to do to dial the car in for qualifying and the race, and it's going to be difficult because the temperature is dipping and the balance is shifting all the time.
"Generally the car has been feeling good this weekend. I feel strong in the car."
Just 0.3secs separated Alonso in third from McLaren's Kevin Magnussen, who was ninth behind Toro Rosso's Daniil Kvyat.
Force India's Sergio Perez was 10th, ahead of Williams' Valtteri Bottas, Toro Rosso's Jean-Eric Vergne, Force India's Nico Hulkenberg and Ferrari's Kimi Raikkonen, who was a second slower than his team-mate, in 14th place.
The Finn continues to struggle with the feel of the front of the car on braking into slow corners, as he has done all season so far.
The two Williams drivers set their fastest times later in the session than the other leading teams, when the conditions were cooler and the track faster.
The Mercedes cars looked considerably faster than the rest of the field whether it be on low-fuel qualifying-type runs or on the race-simulation laps later in the session.
Hamilton and Rosberg appeared to be evenly matched in race trim, with a close fight between the Red Bulls, Force Indias, Williams and Alonso's Ferrari to be the next fastest cars.
BBC F1 analyst Allan McNish said: "Mercedes are again the class of the field. Williams are probably going to be the prime runners for third position.
"Then Force India weren't fast in overall terms but their long runs are super-good - they are good at doing one stop fewer than the rest.
"Then it's going to be the Ferraris, McLarens and Red Bulls, although never count against Red Bull pulling something out of the bag."
"Mercedes are very quick. It's not a surprise," said Red Bull's Vettel. "There is a long way for us to catch up, but long term it is important we make bigger steps than them.
"It's very difficult to know where we are in terms of pace but it is very easy to say we are not with Mercedes."
Mercedes had also been one-two with Hamilton ahead of Rosberg and Alonso third in the earlier first session.
Red Bull had another fuel-flow sensor failure, according to team principal Christian Horner. Almost every failure of that mandatory FIA part this season has happened on a Renault-engined Red Bull, Lotus or Toro Rosso and Renault do not yet know why that is. The only exception is one failure on Sergio Perez's Force India in Australia.
It was a better session from Lotus, whose cars have been plagued by reliability problems this season, but whose drivers Pastor Maldonado and Romain Grosjean completed a full programme and ended up 17th and 18th places behind the Saubers of Adrian Sutil and Esteban Gutierrez.
Marussia's Max Chilton ended his session early with a spin at Turn Four. The team did not immediately know what had caused the problem.