Lewis Hamilton believes Max Verstappen represents stiff competition, both in the opening two Formula 1® races in Austria and in the 2020 season as a whole.
Ferrari have embarked upon a major redesign of their car after struggles in pre-season testing, with Charles Leclerc saying he is "99 per cent certain" they will struggle more than they did last year.
The Italian team's upgrades will not be available until Hungary, but Red Bull are an immediate threat after impressing Mercedes in the pre-season period before the coronavirus lockdown.
Verstappen has won in Austria for each of the last two seasons, and will be optimistic of a fast start when the Red Bull Ring hosts the first two races of a revised 2020 calendar amid the coronavirus pandemic.
"I mean, they looked quite strong through testing," Mercedes driver Hamilton said when asked if Red Bull represented the biggest threat for the new Formula 1® campaign.
"It was quite close between the top three teams in testing - I think Red Bull particularly being the closest to us from our understanding.
"And Max is progressing year on year, so I anticipate they are going to be strong competitors.
"They are always very strong here at this track, so definitely going to be a challenge these two weekends. But I'm excited to see how that goes."
Hamilton has not won in Austria since 2016 and only has a longer streak without victory in Australia of the circuits that were on last year's calendar.
Team-mate Valtteri Bottas triumphed in 2017, before Verstappen's double wins came amid Mercedes woe over the last two years, including major cooling issues in the 2019 race.
"We've not been terrible [since 2017], but it is a challenging circuit with the height of the circuit, the temperatures that we see," added Hamilton.
"We've arrived here [in previous years] and it's usually kind of not in the perfect window of operating temperatures for our car.
"But I'd like to think we're in a better position this year with improvements that we've made for this season's car. So, time will tell."
Bottas was also in an optimistic mood as he looks to push six-time world champion Hamilton closer this year.
"I was pretty happy when I heard there was going to be two races here, I like this track," he said.
"Historically, if you look in the past, I've also had tracks that have not been very successful but I've still been able to be good there.
"It's tricky to say anything from the past - it's a new season after a long break. But I feel I've had good preparations within myself and with the team - so that's why I do feel confident.
"Very soon we'll see where we are as a team and also me personally pace-wise."