Lewis Hamilton led both Friday practice sessions as the world champion put Mercedes back on top at the Chinese Grand Prix.
The Briton posted a 1:37.219 on the soft compound tyres in the afternoon but was only four tenths clear of the Ferrari of Kimi Raikkonen as Nico Rosberg struggled to put a representative lap together.
Daniel Ricciardo put Red Bull in its most promising position so far this season with the third best time having opted for a low drag rear wing to try to negate the power disadvantage down Shanghai's long back straight.
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Ferrari close early gap
In the morning it appeared Mercedes were indeed back as the dominant force after Sebastian Vettel's victory last time out in Malaysia, as Hamilton and Rosberg held a comfortable margin over the two Ferrari's.
But it all changed when serious race set-up work began in the afternoon and the Pirelli tyres again offered the Scuderia hope of challenging the Silver Arrows.
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Over the course of the longer run in the final half an hour Kimi Raikkonen was not only faster but also more durable on the medium compound rubber, which is likely the favoured race tyre given the soft option tyres are only lasting for around 10-12 laps, than Nico Rosberg with the pair even having a small wheel-to-wheel battle out on track.
Vettel and Hamilton also ran close together out on track but errors from the leading Mercedes meant we did not see a true reflection as to how that battle is shaping up.
The straight line speed of the Ferrari is also slightly higher than the Mercedes down the back
straight as they use the same strategy of matching the perceived strength of the Mercedes as they did in Malaysia.
Red Bull back on form?
The arrival of Ricciardo on the scene adds another variable to the mix, while his soft tyre, lower fuel run was done a little later than the rest when the track was slightly more grippy, his long run was also reasonable compared to the top two teams and his tyre preservation on a similar
level to the leading Ferrari in that area.
The biggest issue for the team however, seems to be the brakes. After both cars had problems in Malaysia, Red Bull switched back to a 2014 configuration with Brembo but Daniil Kvyat still had issues as, after a short pit-stop, his left-rear brake overheated and he eventually lost all brakes having a small bump into the wall at the end of the back straight trying to recover to the pits.
Williams' quest for progress hits wall
Lock-ups were a common feature on a cool day and particular into the penultimate corner where drivers brake from nearly 210mph to around 40mph for the hairpin and for Felipe Massa he had perhaps the scariest moment of the day.
The Brazilian locked his rears in the Williams on approach to that turn and slid off the track luckily avoiding the wall on the outside but for a small knock just before he stopped, the incident also caused a brief red flag period.
Overall, Williams had a quiet day as they trialed a new rear suspension set up and front wing in a bid to make up some of the ground they have lost to Mercedes and has seen them be surpassed by Ferrari.
After Massa's off, Valtteri Bottas was the only man running but was seventh quickest behind as the Finn fell behind both Red Bull's.
Improving McLaren join midfield scrap
With Red Bull seemingly hauling themselves out of the first midfield battle, their place has been taken by an improving McLaren team who had Jenson Button 10th and Fernando Alonso 12th.
Top speed remains an issue but the handling on the long sweeping corners seems to suit the MP4-30 and they have joined Sauber, Lotus and Toro Rosso for the minor points places.
A very close battle is brewing with Felipe Nasr, who looks back on form after being out of sorts in
Sepang, Romain Grosjean and Button all covered by less than a quarter of the a second in the final three top 10 places and only nine-tenths covering the eight drivers.
Force India and Manor left behind
Force India are still languishing at the bottom of the midfield with only Manor Marussia behind them as Roberto Merhi led Will Stevens with the Briton having power unit problems in the afternoon.
Back at the front, though and it seems those who believed the unique heat and humidity of Malaysia was the reason for Ferrari's competitiveness appear to have been proved wrong.
Yes, over a single lap the Mercedes is still the fastest car by a reasonable margin but when it comes to maintaining that pace over a 305km race, Ferrari may just have Mercedes' number again.
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