Il Nuovo Postiglione - Mercedes have 'taken a step back': Russell

Russell and Mercedes have long been tipped as title candidates for this season as all the teams adapt to the rule changes which have led to a complete overhaul of chassis and engines, with a bigger focus on energy management.

The teams had a shakedown in Barcelona at the end of January and return to the track in Bahrain next week ahead of the season opening in Melbourne on March 8.

"Barcelona was very smooth (for Mercedes), and probably smoother than we actually anticipated, in terms of reliability, in terms of performance," said Russell at the press conference after topping the time sheets in the morning session.

"We’ve got to Bahrain and in both regards we’ve taken a step back.

The 27-year-old, who finished fourth last season, switched the spotlight from his own team to Red Bull.

"I do think this test has been a bit of a reality check for all of us," he said.

"The truth is, Red Bull in Barcelona, day one, hit the ground running, were well ahead of all of their competitors – ourselves, Ferrari and the others.

"Day one here in Bahrain, again, they sort of looked on par. At the moment, they’re very much the team to beat."

Russell's comments came a day after Red Bull's four-time world champion Max Verstappen slammed the rule changes which, he said, made the cars feel like "Formula E on steroids".

- 'Reliable, powerful' -

Vertappen's new teammate Isack Hadjar, promoted from the Racing Bulls team, struck a brighter tone when he described his team's work in Bahrain as "better than we hoped for".

The 21-year-old spent much of Friday morning in the garage as the mechanics worked on his car but ended the day with 87 laps under his belt and the fifth quickest time.

"Very surprised already from the Barcelona shakedown to here, like how many laps we managed to achieve," he said.

Hadjar also gave the thumbs up to Red Bull's factory-developed power unit, no longer the Honda engine of previous years although benefitting from some input from Ford.

"The power unit seems so far reliable, powerful, so I'm very happy so far.

"If anything it's going a bit better than we hoped for, more on the PU side."

Seven-time world champion Lewis Hamilton produced the third quickest time of the day in his Ferrari before stopping on the track near the end.

The new Cadillac team also had an issue when Valtteri Bottas brought out the first red flags of the day after stopping out on track, just as Sergio Perez did on Thursday.

The new Adrian Newey-designed Aston Martin had a difficult testing, clocking just 206 laps across the three days.

“We’re still learning the car, the engine, but we have a lot of work to do, and catching up to do," said Lance Stroll.

“Right now, we look like we're four seconds off the top teams, four-and-a-half seconds."

Testing resumes in Bahrain on Wednesday.

F.S.Ferrari--INP