Valtteri Bottas set the pace on the second day of F1 Testing as Mercedes rebounded from their slow start in Bahrain, edging AlphaTauri, Aston Martin and McLaren in a competitive and increasingly fast closing session.
After their surprising reliability-affected opening day of 2021, Mercedes - despite Lewis Hamilton's spin into the gravel before lunch - improved throughout Saturday's running, completing over 100 laps while also taking their more familiar position at the top of the timesheets thanks to Bottas' low-fuel soft-tyre lap late in the day.
"It's been the best half day for Mercedes at testing, by some margin," said Sky F1's Martin Brundle. "It's just beginning to come back under control again."
Bottas' 1:30.289, the fastest lap of the test so far, was a tenth of a second quicker than Pierre Gasly and Lance Stroll - who helped Aston Martin bounce back from a disrupted morning - while McLaren were also in the mix all day.
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Star signing Daniel Ricciardo topped the timesheets in the morning, and Lando Norris finished only 0.3s down on Bottas with his new Mercedes power as the pace increased under the lights.
Antonio Giovinazzi, quicker than Charles Leclerc's works Ferrari in his Alfa Romeo, also impressed - while Fernando Alonso completed 128 laps on his first F1 session in two years, and less than a month after Alpine's returning two-time world champion broke his jaw in a cycling accident.
Conspicuous by their absence at the front were Red Bull, who didn't post any headline-grabbing times as Sergio Perez. racked up plenty of mileage on his first official day in the RB16B, lost track time at the end of the day after a bizarre stoppage, losing parts of his engine cover on the main straight.
Assessing Mercedes' day and the other talking points
Considering Mercedes have won F1's titles in each of the past seven seasons, their return to the top of the timesheets would often be an ominous sign for their rivals. But Bahrain testing has so far been too close to predict any kind of a pecking order - while Mercedes' Saturday was hardly plain-sailing, either.
Hamilton said he was struggling to find the "sweet spot" of his new W12 car after beaching it in the gravel in the morning session, with the seven-time world champion finishing the day a lowly 15th on the timesheets.
"It's Day Two of testing so we're just focused on doing our job," said Hamilton, whose rare error caused a red flag. "We're focused on trying to understand the car, there's no point being worried just yet."
He was ahead of only another multi world champion Sebastian Vettel, who completed 10 laps before swapping out of the car at lunch due to a gearbox issue on his new Aston Martin.
The expected midfield teams took centre stage in the afternoon session as Norris, Gasly and Giovinazzi consistently traded fastest times - to be joined and passed by Bottas on his qualifying-esque run.
Alonso, who enjoyed a remarkably consistent first day in his Alpine car, didn't fit the soft stars for a quicker lap - nor did Perez, as Red Bull, who were fastest on Day One, kept their cards close to their chest.
That sets up an intriguing final day of a shortened pre-season testing on Sunday, before the sport's 2021 season kicks off at the same Sakhir track in two weeks.
Other notable times on Saturday were Nicholas Latifi's in the Williams - just 1.3s off the Mercedes - while Yuki Tsunoda impressed in the morning for AlphaTauri.
Mick Schumacher, meanwhile, completed a remarkable 88 laps in just four hours in his Haas.