Carlos Sainz Jr. feels McLaren needs to "fear everyone" in Formula 1's midfield fight heading into next season after seeing a number of teams make significant steps in performance through 2018.

Sainz will join McLaren from Renault for 2019, with the British team hopeful of improving on its sixth-place finish in the 2018 constructors' championship.

Top 10 of F1 2018 Moments

However, McLaren only gained one place from Force India after the team - now known as Racing Point - was forced to re-enter the championship mid-season, and struggled to beat teams such as Sauber and Toro Rosso on occasion.

Sauber's revival was one of the particular surprises in F1's midfield through 2018 as it rose from the back of the grid to often lead the midfield runners by the end of the year.

While Sainz highlighted Sauber's form, he also said the threat posed by the other midfield teams meant McLaren had to be wary of everyone heading into the new season.

"If [Sauber] are actually now the strongest midfield team, I think we should all fear them. But next year we should fear everyone," Sainz said.

"You think about Sauber, but then you think about Force India with a bit more budget, what they could be capable of. You think about Renault, what they are able to do. You think about Haas, if they keep getting the amount of Ferrari parts that they are getting and Ferrari keeps dominating, they should be also very strong.

"You think about McLaren with a whole new project and wanting to invest and wanting to go back to P4 in the championship after a difficult year.

"You think about all these teams, and you think anyone could lead the midfield battle next year."