Reigning Formula 1 World Champion Lewis Hamilton has not enjoyed the best of preparations ahead of the 2021 campaign.

His Mercedes team have completed fewer miles than any other constructor during the three days of pre-season testing in Bahrain - with Hamilton himself involved in several spins that left him stuck in the gravel.

Bidding to land the eighth Drivers' Championship of his career, Hamilton has admitted that he expects competition on the grid to be strong this term.

In particular, the Brit believes that Red Bull can pose a serious challenge to Mercedes' recent dominance.

Currently second-favorites with the bookies for this season's Constructors Championship, Hamilton is under no illusions as to the threat Red Bull could offer this term.

"They can be favourites if they want. I mean, they're looking strong," conceded Hamilton following the final day of testing, per thesun.co.uk.

"I don't really focus on that kind of stuff and they've had some really good running."

Hamilton also had words of praise for Red Bull drivers Max Verstappen and new arrival Sergio Perez, declaring that the pairing has turned Christian Horner's team into 'a different animal' this year.

"Both the drivers have been looking quite strong. They're going to be a different machine or animal this year with a really good, strong line-up for two drivers and a really good car."

Verstappen underlined his team's credentials by setting the quickest overall time in testing.

However, Red Bull adviser Helmut Marko refused to get carried away when discussing his side's chances of springing an upset in the season-opening Bahrain Grand Prix next weekend.

"That was certainly the best test since Red Bull Racing came into existence," said Marko. 

"Everything worked right from the start but we know that Mercedes is the favourite, and I assume that they drove with much more fuel than we did.

"Mercedes is the favourite and we are the first challenger. That's how I see the situation," revealed Marko. Behind them, it's going to be a battle.

"I think McLaren will stand out a bit, but not at the level of Mercedes and us. The midfield is certainly closer together."

Given how poorly pre-season testing went for Hamilton, it is no surprise that he would have liked more time to work with his car ahead of the first race of the new season.

With that said, he has dealt with similar adversity in the past and typically found to way to work through it.

We will see whether Red Bull's form in testing carries over to a race proper in less than two weeks when the new season gets underway.