Mercedes technical director James Allison says the Formula 1 champions fear both Red Bull and Ferrari for 2020 and are unsure if any of next season's 22 grands prix will be a sure bet.
The Silver Arrows are basking in the glory of a record-breaking sixth-straight drivers' and constructors' title double in 2019, with Lewis Hamilton having also collected his sixth individual crown.
After a stunning start to the 2019 campaign, which saw Mercedes take five straight one-twos to begin the year, both Ferrari and Red Bull closed the gap over the campaign.
And with stable regulations for 2020, the prospects of a genuine three-pronged fight for supremacy has fans excited, and the top dogs of F1 concerned.
"We don't just have to fear Ferrari," Allison told Auto Motor und Sport. "Red Bull is also an extremely strong team.
"We don't go into a single race with the confidence that we will win.
"It is clear to us that between today and Melbourne 2020 we have to make our car a lot faster if we want to have a chance against them. But this task is not meant to be easy. It is the ultimate challenge."
Mercedes managed to maintain their success on race day in the second half of 2019 despite Ferrari suddenly gaining a huge advantage in qualifying, which saw them take six pole positions immediately after the summer break.
However, Allison says the Silver Arrows cannot rest on their laurels in the hope that greater race pace will again come their way in 2020.
"Speed in the race is an important factor, because you can put pressure on your opponent to make mistakes," he said.
"If you have a faster car in your opponent's gearbox, he doesn't have many good options.
"The advantage of the faster car in the race gives you many strategic advantages.
"Of course, we would be happier if we had racing speed and could start from pole position. That must be our goal next year."