On the basis of three short days of preseason testing in Bahrain, Mercedes' Formula One dominance is under threat.
Seven straight years of title success warn against making such statements at this time of the year, but if you focus purely on what happened on track in Bahrain, it's hard not to come to the conclusion that Red Bull has built a faster car.
"We can see from the data we've collected over the last few days that on race pace, we're not as quick as Red Bull," Mercedes' trackside engineering director Andrew Shovlin said on Sunday evening.
"The lower fuel work was a more confusing picture, we didn't gain enough and we need to go and look at our approach as far too many cars were ahead of us on pace today."
It shouldn't be a surprise that Red Bull has produced a car capable of competing with Mercedes.
The former championship-winning team lost its way at the start of F1's turbo-hybrid era in 2014, but has always had the staff and facilities back at base to return to the front.
After a slow start to the season last year, Red Bull made significant progress with its car concept in the latter half of 2020 and beat Mercedes fair and square at the final race of the season in Abu Dhabi.
With cost-saving regulations requiring more carryover of car parts from 2020 to 2021 than normal and engine-supplier Honda pulling out all the stops ahead of its exit from F1 at the end of the season, a competitive Red Bull package was to be expected this year.
What was surprising during testing was the apparent step backward made by Mercedes.
On Sunday evening when track conditions were best, Lewis Hamilton's fastest lap was 1.065s off Max Verstappen's best effort despite the Mercedes driver setting his lap on a tyre compound that should have been 0.3s faster.
Mercedes often hides its true pace in testing by running heavy with fuel, but even allowing for a 20kg discrepancy between the two cars, the tyre- and fuel-corrected gap would still be a whopping 0.7s.
What's more, Carlos Sainz in the Ferrari and Yuki Tsunoda in the AlphaTauri also set times on Sunday evening, which, based on fuel-load and tyre corrections, would be 0.35s and 0.15s faster than Hamilton's lap respectively.
Perhaps the surprising pace of the Ferrari and AlphaTauri should act as a warning about reading into the lap times, but even with a significant margin for error applied, Mercedes has clearly lost some of the single-lap prowess that defined its 2020 season.
When comparing testing performances with those of rivals at F1's traditional preseason venue, the Circuit de Catalunya, teams usually allow for an error band of 0.4s to 0.5s, but in Bahrain the track conditions widened that error band significantly.
Changing wind conditions had the potential to account for a swing of up to 0.5s in lap time from one run to the next and the track was as much as 1.5s per lap quicker once the sun had gone down in the evening.
What's more, Bahrain is a power-sensitive circuit, meaning the choice of engine mode -- which only the team will know -- was more critical to performance than it would be in Barcelona.
Put simply, it's very unlikely Sunday's 1.065s lap time difference between Verstappen and Hamilton is a true representation of the gap between the two cars, but for Mercedes to be ahead of Red Bull would require a lot of caveats to be at play.
Above all else, it's clear Mercedes' new car did not meet expectations in testing and the team is now in a race against time to understand why ahead of qualifying for the Bahrain Grand Prix on March 27.
What went wrong for Mercedes?
The three days of testing got off to a bad start for Mercedes when Valtteri Bottas reported a gearshift issue on his first lap out of the pits. In the time it took to fit a new gearbox to the car, Mercedes sacrificed around 60 laps to its main rivals and it failed to recover that mileage over the following two and half days.
In a complete reversal from preseason tests in recent years, Mercedes finished the week at the bottom of the mileage charts with 304 laps, compared to the 369 laps of Red Bull and the 422 of the two most active teams, Alfa Romeo and AlphaTauri.
But unlike previous years, Mercedes said maximising its lap count was not one of its main objectives in Bahrain.
With the carryover of so many parts from 2020 to 2021 to comply with cost-saving regulations, a certain level of reliability was assumed (despite the gearbox issue) and Mercedes believed its time would be better spent conducting performance-based experiments than running the car to breaking point.
What's more, Bahrain is a circuit that induces high rear tyre degradation and running lengthily stints would have skewed the experiments due to the drop off in performance of the rear tyres. Better to get batches of solid data over shorter runs than a race simulation where tyre degradation becomes the main factor influencing performance.
As a result, it was always the plan for Mercedes to limit its mileage compared to previous years and focus on gaining as much quality data as possible from the 32 sets of tyres and three days of testing available.
But as the team set about its programme following Friday's gearbox problem, it soon became clear that the rear of the car was unstable and the drivers were struggling to extract performance from it.
"We're still not fully comfortable [with the car]," Bottas said on Sunday. "We are going forward with the set-up of the car and the understanding of the car and getting it to behave better, but there is still more work to do.
"So we will keep working, we will keep trying to get the balance better and get the car to behave nicer and hopefully that way, be faster."
It raises the question of how a dominant, well-behaved car like last year's W11 morphed into the tricky, unpredictable W12 seen in testing.
Mercedes didn't have an answer to that question on Sunday evening and will spend the next 11 days before the first practice session in Bahrain poring over its data to better understand it.
But with the car's chassis and the majority of its mechanical components being carried over from last year's championship-winning car, the logical conclusion is that the issue is linked to the team's interpretation of 2021's new aerodynamic regulations.
Ironically, the changes to the rules were introduced by the FIA as a safety measure following two tyre failures on Mercedes' cars at the British Grand Prix.
The current generation of F1 cars are faster and heavier than any other in the sport's 70-year history, and there were concerns that the ever increasing stresses going through the tyres would result in more failures if car development was left unchecked.
Therefore, the governing body attempted to peg back downforce levels by targeting changes to three main areas: cutting away a triangular section of the floor ahead of the rear tyres, reducing the length of strakes on the diffuser and limiting the way in which rear brake ducts were being used to generate downforce.
Alone, the individual changes wouldn't amount to much but collectively they aimed to strip 10 percent of downforce from last year's cars.
Naturally, the teams have done everything they can to minimise the loss in performance over the winter and, based on the last three days of testing, tyre manufacturer Pirelli believes a more realistic figure is a four-to-five percent reduction in downforce.
The areas targeted by the regulation changes are particularly sensitive to the creation of downforce at the rear of the car and it may be the case that Mercedes' incredibly refined 2020 aero package was hardest hit.
What's more, it's not uncommon for gains in the wind tunnel to produce unwanted side effects on track, especially when conditions -- such as the high winds seen in Bahrain -- can't be replicated at the factory.
The team says the instability was present regardless of the wind speed and direction, hinting at a more fundamental issue, but it would likely have made more progress in understanding the car if the conditions had been stable.
It also reported that the issue was less obvious on heavy fuel, with Mercedes able to get closer to Red Bull's level of performance and retain a more familiar gap to the midfield teams when focusing on race pace.
Advantage Red Bull?
The last time a team other than Mercedes was the favourite to win the first race, it did not go to plan.
After preseason testing in 2019, the paddock was convinced Ferrari had the fastest car but when F1 turned up for the first race in Australia it was Mercedes that dominated.
While it was true that Mercedes had struggled during the first week of testing in 2019, its development curve was far steeper than Ferrari's, and by the time the opening race rolled round it had resumed its place at the front while the Melbourne circuit exposed previously-unseen weaknesses on Ferrari's car.
The situation this year is different as the first race is due to be held at the same venue as the test, and the tell-tale signs of improvement Mercedes showed on low fuel at the end of the 2019 preseason were not present in Bahrain -- if anything it was the opposite.
What's more, the development curve of this year's cars is not expected to be as steep as in the past. Development focus has already shifted to the regulation overhaul coming in 2022 and the introduction of a budget cap and new wind tunnel restrictions this year means there is less resource to throw at troubleshooting the 2021 design.
The group of engineers at Mercedes have proved incredibly adept at problem solving in recent years and the team is optimistic it will uncover more performance ahead of the first race by understanding the data it has gathered.
"We will crunch the data to understand where we performed well and where we did not, where we had good correlation to our simulations in the [wind] tunnel and where not," team boss Toto Wolff said on Sunday.
"It's like sleeping on an idea; the next day you wake up more intelligent."
It would be brave to bet against Mercedes at this stage, but it's also fair to say the team has not had such a difficult preseason ever since its period of dominance began in 2014.
What's more, Mercedes issues should not detract from the strong performance of Red Bull in Bahrain, and even if the world champions find a solution, it's unlikely their main rivals will stand still in the development of the RB16B.
The combination of the two could be enough to tip the balance as the 2021 season gets underway and will certainly make for a fascinating start to the season.