Lewis Hamilton's former teammate Nico Rosberg says both Sebastian Vettel and Valtteri Bottas have the potential to challenge the world champion this year.
Rosberg beat Hamilton to the title in 2016 before retiring from the sport as champion at the end of the year. Bottas replaced Rosberg at Mercedes but for the last two years Ferrari's Vettel has been Hamilton's closest competitor.
Rosberg believes Vettel is under intense pressure to mount a successful title challenge but says he is capable of getting the better of Hamilton over the course of a season.
"The pressure is on, absolutely," Rosberg told Sky Sports. "He was feeling it, I'm sure, already last year and especially driving for Ferrari that's where the pressure is the highest because you've got the whole country on you.
"He's feeling it, that's clear. He just needs to put it together next year.
"He [Vettel] needs to improve and the team needs to improve overall. They just had too many mistakes last year because possibly they should have won it."
Rosberg beat Hamilton to the title after an intense rivalry as Mercedes teammates in which Hamilton won titles in 2014 and 2015. Bottas' arrival at the team in 2017 brought about a more relaxed dynamic at Mercedes, but Rosberg believes his successor needs to hit the ground running this season and "annoy" Hamilton in the opening rounds.
"Valtteri, I think is a lot better than he has been able to show recently," Rosberg added. "So with a new season starting again he has a much better chance of establishing himself in a stronger position and even really annoying Lewis quite a lot, I think that's possible.
"It always depends on how the season starts. Last year he had quite a lot of bad luck as well, but if the luck is on his side and he starts well it could be a whole different situation."
Testing for the new Formula One season gets underway on February 18, with the first race taking place on March 17 in Australia.