Sebastian Vettel set the pace in first practice as Ferrari hinted they could be the team to beat at the Chinese Grand Prix this weekend.
Vettel finished the first session 0.207 seconds ahead of Mercedes' Lewis Hamilton - but the German was running harder tyres which should be slower.
Team-mate Charles Leclerc was third ahead of Red Bull's Max Verstappen.
Mercedes' Valtteri Bottas was fifth ahead of Renault's Daniel Ricciardo and Red Bull's Pierre Gasly.
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As in Bahrain two weeks ago, Ferrari's advantage over Mercedes appeared mainly to be on the straights.
Bottas heads into the weekend as the championship leader, after a win in the season-opening race in Australia and a second behind Hamilton in Bahrain.
But Hamilton's win in the desert came against the run of play after Leclerc, who had dominated the weekend, suffered an engine problem in the closing laps.
There remains uncertainty about the shape of the competitive picture, after two races of which Mercedes and Ferrari have dominated one each.
In theory, there is at least 0.5secs between the tyre compounds, so for Vettel to be 0.2secs quicker than Hamilton despite using a one grade harder tyre appears to bode well for Ferrari.
And the internal dynamics at the Italian team are a major talking point - Vettel needs to bounce back in China to protect his position as nominal team leader after Leclerc's impressive performance in Bahrain.
The session was relatively incident-free on a sunny day in smoggy Shanghai, although Racing Point's Lance Stroll had a spin out of the final corner early in the session.
But the Canadian was an impressive ninth, just behind Toro Rosso's Daniil Kvyat and ahead of the Haas cars of Romain Grosjean and Kevin Magnussen.