Lewis Hamilton headed Mercedes team-mate Nico Rosberg in second practice at the Canadian Grand Prix.
The two Mercedes were separated by 0.175 seconds and Hamilton was 0.455secs clear of Red Bull's Sebastian Vettel in third ahead of the two Ferraris.
Kimi Raikkonen was fourth, 0.053secs quicker than Fernando Alonso.
Williams' Felipe Massa and Valtteri Bottas were next ahead of McLaren's Kevin Magnussen and Jenson Button.
Red Bull appeared to be struggling until Vettel popped up from 15th into third with 20 minutes to go, an early gearbox problem meaning he had to run the faster 'super-soft' tyres later than his rivals when the track would have more grip.
Red Bull team principal Christian Horner said: "It was a very good lap from Seb. He's still trying to dial his way into the circuit on set-up but he put a quick lap together there."
The world champion's team-mate Daniel Ricciardo was 12th fastest, behind Toro Rosso's Jean-Eric Vergne and Lotus's Romain Grosjean.
The long straights of the Circuit Gilles Villeneuve penalise the lack of power of Red Bull's Renault engine compared to the dominant Mercedes power-unit, which is also used by Williams, McLaren and Force India.
As a result, Red Bull are expecting a difficult weekend. Horner told BBC Sport that somewhere around fifth place was a realistic target for them in Canada.
Horner said that the Mercedes' race pace during the simulation runs later in the session was "a bit depressing, but it's going to get warmer over the weekend. That's probably going to have a big influence so this might not be that relevant to Sunday."
Mercedes, meanwhile, are expected to continue their domination on a track that could have been made for their car, which has a compliant ride for taking the kerbs at the chicanes, good mechanical grip for traction out of the slow corners and strong power.
Hamilton, who was second fastest to Alonso in first practice, was faster than Rosberg on both the harder 'soft' tyre and the 'super-soft'.
Hamilton was just 0.085secs ahead on the 'soft' tyre early in the session. When they switched to the super-soft to try a qualifying-type run mid-way through the session, Hamilton's first lap was 0.569secs ahead of Rosberg, who ran wide at the hairpin on his attempt.
The German had another go after a 'cool-down' lap and closed in.
The battle between the two seems finely poised, as despite the almost 0.2secs margin on ultimate time, Rosberg's tyres would have lost their edge by the time he set his best lap.
On the race-simulation runs, Hamilton appeared to have a clear advantage of about 0.15secs on the super-soft tyres, although the two lapped at an almost identical pace on the softs.
Ferrari appear to be the second fastest team - they have taken a package of upgrades to Canada, including aerodynamic revisions among which are a new floor and engine performance improvements.
The two drivers shared the development work. Alonso ran the new floor in the first session, when he was 1.34secs clear of Raikkonen, before it was switched to the Finn's car for a comparison in the second session, when Alonso ran an older design.
As usual, there was spectacular viewing in Montreal as several drivers had off-track moments as they struggled to find the limits during braking for the difficult chicanes.
Among them were Sauber's Esteban Gutierrez and Lotus's Pastor Maldonado, who both had very scruffy sessions.
At the back of the grid, Caterham's Marcus Ericsson and Marussia's Jules Bianchi both had troubled sessions.
Ericsson pulled off after just half an hour following a transmission problem and Bianchi managed just two laps with what is believed to have been a power-unit problem.
Toro Rosso's Daniil Kvyat similarly struggled, completing only nine laps.