Chinese GP, F1's 1,000th race: Bottas fastest in practice
Mercedes driver Valtteri Bottas headed Ferrari's Sebastian Vettel by just 0.027 seconds in second practice at the Chinese Grand Prix.
Max Verstappen's Red Bull was third fastest, 0.221secs off the pace despite running wide at the final corner.
Lewis Hamilton's Mercedes was fourth, 0.707secs off, while Renault's Nico Hulkenberg and McLaren's Carlos Sainz made it five teams in the top six.
Ferrari's Charles Leclerc was seventh after problems on his quick laps.
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Leclerc split the McLarens, with British rookie Lando Norris eighth, ahead of Renault's Daniel Ricciardo and Red Bull's Pierre Gasly.
Vettel's 0.801secs advantage over his team-mate was exactly what the German needed after the Bahrain Grand Prix, where Leclerc dominated and was on course for a maiden victory before suffering engine problems late in the race.
Leclerc's performance in Bahrain raised questions about Ferrari's policy of favouring Vettel in marginal calls at the start of the season.
But team boss Mattia Binotto on Friday reiterated that Vettel remained the team's priority, saying: "If there is any 50-50 situation where we need to take a decision, the advantage would have been given to Sebastian simply because Sebastian has got most of the experience with the team in F1."
Leclerc aborted his first quick lap on the qualifying simulation runs and when he did finally try again after a number of slow laps he was not able to approach Vettel's pace.
Leclerc also ran into problems later in the session - he was unable to do his race-simulation run because of a problem with the cooling system.
It is the third consecutive race in which the engine at Ferrari has caused a concern of one kind or another.
Before Bahrain, Ferrari had to run their engine at a lower performance level in the first race in Australia because of unspecified problems.
And Ferrari changed the control electronics on their cars for this race to try to prevent a repeat of the short circuit in an injection control unit that caused Leclerc's problems in Bahrain. Teams can use only two control electronics boxes for an entire season, so Ferrari have now used up their allocation.
Mercedes felt they were on the back foot after Vettel headed the first practice session, but Bottas was able to usurp the German at the top of the times in the second session, when teams come close to simulating qualifying conditions.
And Bottas also appeared to have a slight performance advantage in the race runs later in the session.
However, both Mercedes drivers had pretty much identical spins early in the session, when rounding the interconnected Turns One and Two just after exiting the pits.
Hamilton said: "It was quite cold out there, which makes it difficult for the tyres. I was struggling with the car, so we've got work to do, particularly on my side as Valtteri looked much more comfortable.
"It is nice to see how close it is between us, Red Bull and Ferrari."
Haas driver Romain Grosjean, who was 17th fastest, suffered a front wing failure during the second session, when the right-hand side of the wing broke at the point where the central mandated neutral section meets the outer parts that teams can manipulate for aerodynamic effect.
Off track, a number of teams are already running into reliability issues with engines.
Renault have fitted updated MGU-K units - the part of the hybrid system that recovers energy from the rear axle - to both factory cars of Nico Hulkenberg and Daniel Ricciardo as well as to the McLaren of Lando Norris, after the other McLaren driver Carlos Sainz had one in Bahrain.
This is because of repeated failures in the new design of MGU-K that was introduced for this season.
And Toro Rosso's Daniil Kvyat has become the first Honda driver to take new engine parts - he has a new internal combustion engine, turbo, MGU-K and MGU-H (the hybrid part attached to the turbo) after a failure in first practice.
Hulkenberg, Ricciardo, Norris, Sainz and Kvyat are as a result likely to run into grid penalties later in the season as each driver is restricted to just three engines, turbos and MGU-Hs and two MGU-Ks and control electronics per season.