Lewis Hamilton headed a Mercedes one-two in second practice at the season-opening Australian Grand Prix.
Hamilton beat Valtteri Bottas by 0.048 seconds but the two were 0.8secs clear of the two Red Bulls of Max Verstappen and Pierre Gasly.
Ferrari, who appeared to have the quickest car in pre-season testing, were fourth with Sebastian Vettel.
The suspicion is that the Italian team were hiding their true pace before qualifying on Saturday afternoon.
Vettel was 0.8secs slower than the Mercedes on both short, qualifying-style runs and longer race-style runs, which is completely contradictory to their form in testing, when all their rivals felt Ferrari were ahead.
- Hamilton fastest in Australian GP first practice
- Hamilton, Vettel and Button pay to tribute to 'iconic' Whiting
- With the new season here, we look at the five key talking points
- 5 Live F1 podcast: Australian Grand Prix preview
Ferrari playing games?
The session left plenty of intrigue ahead of qualifying on Saturday, when the cars' true pace will be shown for the first time.
Vettel ended the day 0.873secs slower than Hamilton, a result that, if it were repeated on Saturday afternoon would surprise the whole paddock.
Vettel was just 0.03secs slower than Hamilton in the first session of the day, as 0.084secs covered them and Verstappen.
But while Mercedes found a second's worth of pace for the second session, Vettel improved by just 0.164secs and Verstappen by only 0.392secs.
Verstappen's relatively slow time can be explained by the fact he set his fastest lap at the start of a long run, so it cannot be compared with other drivers' headline times because of the amount of fuel he would have had on board.
Adding to the confusion, on the race-distance runs late in second practice, Vettel on the fastest 'soft' tyre was only very slightly faster than team-mate Charles Leclerc on the slowest 'hard'.
Alfa Romeo's Kimi Raikkonen was fifth quickest - just 0.099secs behind his former Ferrari team-mate Vettel.
That margin added to the suspicion that Ferrari might have been playing games because the margin between Vettel and Raikkonen was smaller than it was on average between them when they were team-mates. So for Raikkonen to be that close in a car that will undoubtedly be slower only emphasised the point that Ferrari can find plenty more time.
However, Vettel said he had not been happy with the Ferrari's behaviour on track.
Vettel said: "It was a tricky day for us, we are not there yet where we would like to be, not in terms of lap time but more in terms of car feel and handling.
"We didn't really get into the groove and rhythm and that is quite important around here. If we can get that confidence and groove back there is a lot of time to find around here."
Asked about Mercedes' pace, he said: "They looked really quick and I don't know if it was just us struggling a bit today but they were in a league of their own. So maybe everyone else was struggling and we can turn it around bit they look certainly very strong and much stronger than they claimed before the weekend. I am optimistic because I know the car can do much better."
Renault's Nico Hulkenberg and Daniel Ricciardo were seventh and eighth, closely matched after the Australian had a tricky day with a number of delays, first due to a technical problem and then the need to adjust his seat to reduce discomfort in the car.
Leclerc was only ninth fastest, ahead of the Haas of Romain Grosjean.
British novice Lando Norris was 18th fastest in his McLaren, 0.6secs and four places behind team-mate Carlos Sainz.
For Williams, it looks like being a long season. British rookie George Russell and team-mate Robert Kubica, returning after eight years away following the rally crash that left him with life-changing injuries, were 19th and 20th.
Russell was 0.2secs ahead of the Pole - but they were nearly two seconds slower than Norris.
The other rookie, British-born Thai Alexander Albon, had a difficult first day. He crashed lightly in the first session, went off track twice in the second, and ended the day 0.7secs slower than team-mate Daniil Kvyat, who was 11th with Albon 17th.