After seven years of supremacy and a season of easy dominance, Mercedes boss Toto Wolff admitted on Friday that he expects a "dog fight" with rivals Red Bull at this weekend's season-opening Bahrain Grand Prix.
The Austrian team chief, who has overseen seven consecutive constructors and drivers title doubles, had hoped that urgent improvements to the team's 2021 cars would put them on terms in Friday's opening practice sessions.
But instead it was Red Bull's flying Dutchman Max Verstappen who twice topped the times to prove he is the fastest man ahead of Sunday's opening race.
Defending seven-time champion Lewis Hamilton was only third in hot, dry and windy conditions at the Bahrain International Circuit on Friday, beaten by both Vertstappen and fellow-Briton Lando Norris in a Mercedes-powered McLaren.
"The night has fallen and the conditions haven't come towards us," said Wolff, after the floodlit second practice session.
"It's a real dog fight. When you overlay the fastest laps and even the long runs, it's just so very close that every kilogram in fuel load can have a very big swing so we don't really know.
"We are definitely closer here than we were in testing, but I wouldn't know where to position us versus Red Bull."
Hamilton said Red Bull had proved their superior speed and added that he feared the forecast windy conditions due over the weekend while team-mate Valtteri Bottas, who was fifth, said he found his car to be "undriveable at times".
Mercedes' fall from clear supremacy to close contenders followed a set of rule changes designed to reduce downforce and make the cars marginally slower and safer.
In Mercedes case, the aerodynamic rule changes which required cutting away part of the floor of the car in front of the rear wheels had resulted in a huge loss of performance.