'It is not so much that you lose the speed. You just lose the need'

There is an argument that Nico Rosberg's extraordinary retirement represents a more significant achievement than the world title.

It means he stays forever a world champion, because he can never lose it now in racing competition. It is a bold, classy decision, taken quickly.

It was one of the most astonishing announcements in sport as Rosberg on Friday said he was retiring from Formula One at the age of 31, five days after winning the world title.

The Mercedes driver explained that he had "climbed a mountain" that left him "1000% sure" that he should take his bow at the pinnacle of his profession.

It was a stunning revelation when the tense battles with Lewis Hamilton at Mercedes this season had looked poised to continue for years.

But Rosberg resolved that he had sacrificed enough, arguing that he was stepping away from the sport for the benefit of his wife, Vivian, and their baby daughter, Alaïa.

"I'm looking forward to being a husband and a dad for a while," he said.

There will be some, Hamilton one of them, who will feel robbed of the opportunity to take the title from him next year.

Hamilton is in the ring, ready to fight, and his biggest challenger is not turning up any longer.

But there is no handbook for how long a driver should race, and we should remember that he did start in Formula One when he was 19.

I was certainly mentally tired at the end of my career. I started slightly later, finished when I was a fraction older, and second in the championship was as good as I managed. Rosberg, on the other hand, has completed the journey.

Undoubtedly, everybody has been caught by surprise. Quite clearly, Nico had the conversation with Toto Wolff, the head of motorsport at Mercedes, only after considerable reflection.

It was not that he had a vague feeling about what he wanted to do, but that he had already decided.

When you are faced with that as a team principal, it does not matter what contracts are in place. You simply make a settlement.

I recall a similar situation with Mika Hakkinen, my former team-mate at McLaren. Mika was young when he retired, but he was just mentally burnt out after he won his second title in 1999. He then regretted it for a while afterwards and toyed with the idea of coming back.

I get the sense that Nico, having effectively been a career racing driver since he was a little kid in karting, understands the step he is taking in terms of his business structure. He is a bright cookie. So, he is moving on.

He appreciates that this world title, accomplished at the climax of a relentless campaign, is as good as it gets.

Yes, he could win another one, but he recognises how close the battle with Hamilton has been throughout. Against expectations, everything has come together for him.

There is no question that a Lewis Hamilton focused and on form is the finest sight in Formula One at the moment.

Rosberg is man enough to recognise that Hamilton is one of the all-time greats. History will still record Nico as a world champion, too, and he certainly has all the attributes of a very fast, highly successful professional driver.

But you just have to look at the team principals' top 10 drivers and Rosberg would not even be in the top three.

In my own experience, I felt that the time was up at the beginning of the 2008 season, testing the new car at Red Bull.

I had driven winning cars before and been involved in the rebuilding of them. With a new car, you should be full of hopes and dreams, but I saw that this one was not quite going to produce the wins. All of a sudden, I just felt spent.

Even before the season started, the belief I had always had that every year would be better than the one before just was not there. The wind had gone out of my sails.

Once you have that feeling, you have essentially mentally retired, whether you have admitted it to yourself or not.

It is not so much that you lose the speed. You just lose the need.

The likelihood is that Rosberg will be replaced by Pascal Wehrlein, who has come up through the Mercedes young drivers' programme.

He has been groomed to be a Mercedes star. It could be the ultimate opportunity for him.

There is a changing of the guard in F1, and it has happened much quicker than any of us expected.