Mercedes expect to have signed Nico Rosberg's replacement before the end of the year and are looking for someone "better" than the Formula One world champion.

The team's non-executive director Niki Lauda was as surprised as everyone else by Rosberg's bombshell on Friday that he was leaving the sport only five days after winning his first world title.

Lauda told BBC 5 Live's Sportsweek programme: "I was really surprised because normally I think all kind of things can happen but this was never on my radar.

"I spoke to him afterwards to find out because I did this twice in my career, I called him and really wanted to make sure it is not a quick decision that you might regret, and to my question of how sure are you, he said: '1,000 per cent'.

"Then I knew that it is over, you cannot convince him any more. Nico had one aim, to win the championship, and that's it."

Mercedes have dominated the sport for the last three years, with Rosberg narrowly denying team-mate Lewis Hamilton a fourth world title.

There will be many drivers who covet Rosberg's seat but the logistics are complicated, with the top contenders all under contract for next season.

Teenage sensation Max Verstappen - one of only two men other than the Mercedes duo to win a race in 2016 - would surely be top of the list but it is highly unlikely Red Bull would let him go.

If Mercedes want the finished article then former world champions Sebastian Vettel or Fernando Alonso would be obvious choices.

The likes of Pascal Wehrlein, Valtteri Bottas and Esteban Ocon, meanwhile, would offer potential for the future and be easier to secure.

Rosberg ended up on top in his long rivalry with Hamilton (Getty)

Lauda, though, hinted Mercedes are looking for someone who could win races straight away to press home the team's dominance, and they will not waste any time doing so.

The three-time former world champion said of Rosberg: "He's a huge loss because we had the quickest driver set-up over the last three years.

"Nico and Lewis were pushing each other, and now we have to find a better man than Nico because we want to continue to win.

"This is a big problem now for us as a team to find the appropriate replacement. We have to think about it and contact everyone and do proper research on who we're going to put in the best car of Formula One.

"We certainly need a driver for the first test of the new car in February but I can assure you it will happen earlier because whoever it is we have to train them on the simulator, train him into the team.

"I think we should have a decision before the end of the year. We'll give an unexpected Christmas present to one of the drivers."