Mercedes admit they find themselves in a "dogfight" with Red Bull for supremacy at F1's season-opening Bahrain Grand Prix after their rivals set the pace on 2021's opening day.
For the first time in the turbo-hybrid engine regulation era they have dominated since 2014, the seven-time champions topped neither practice session on the opening day of the season - although crucially did admit their W12 performed better than at testing in Bahrain a fortnight ago.
Instead, the honour in both Friday sessions went to Red Bull and Max Verstappen, as that combination appeared to confirm their pre-season testing pace was reflective of the RB16B's potential.
"It's just a real dogfight," said Mercedes team principal Toto Wolff to Sky Sports F1.
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"When you overlay the fastest laps and even the long runs, it's just so very close that every kilogram in fuel load can make quite a big swing. So, we don't really know.
"I think we are definitely closer here than we have been in testing but I wouldn't know where to position us versus Red Bull."
Lewis Hamilton was the quicker Mercedes driver in both sessions, although on short-fuel runs on soft tyres he was also outpaced by the impressive McLaren - now again powered by Mercedes - on the Practice Two timesheet.
"We thought Red Bull would be as fast as they are, if not faster, so we know they are leading at the moment," said Hamilton.
"McLaren are looking great and it's great to see them taking the step. It'll be interesting to see what the long runs are."
However, backing up comments Wolff made to Sky F1 earlier in the day, Hamilton agreed that the W12 was better than at testing, when he spun off twice as both the Briton and Valtteri Bottas struggled with the rear of the car following downforce-trimming rule changes.
"Coming into this weekend we did a lot of work over the past week and I definitely think we've taken a step forward, but we still have more to do," added Hamilton.
The seven-time champion, who surprisingly has only won the season-opening event once at Mercedes back in 2015, added: "This weekend it's looking better but still not perfect. So working as hard as we can. I know we still have a hill to climb but we're staying positive."
Bottas, who claimed the W12 had become "undriveable" towards the end of his long run on worn tyres, admitted: "We still have plenty of work to do if we want to fight for the pole and the win."
What did pacesetter Verstappen say?
The cautious stance Red Bull and their star driver have taken since the start of testing when discussing their potential continued after Friday's running. Verstappen is clearly content with the RB16's pace and consistent handling, but is not getting ahead of himself ahead of qualifying when the true pace of the 2021 cars will only be seen for the first time.
"It was alright. Different conditions compared to testing but a good feeling in the car," Verstappen told Sky Sports. "A few things to look into but overall I'm pretty happy.
"But still tomorrow is a different day, even more windy which in these cars is even more tricky."
When pointed out to him that the newly Merc-powered McLaren also outpaced the world champions' works team, the 23-year-old replied: "I still expect Mercedes to be the strongest Mercedes-powered car."