Not since Nigel Mansell (1992) and Alain Prost (1993) has a freshly-crowned world champion quit F1 - and neither of them did so entirely by choice. 

Nico Rosberg, on the other hand, had a 2017 contract with the Mercedes team he's helped develop since the Stuttgart company bought the outfit in 2010. So why give it up, especially considering the car's utter dominance over the past three seasons?

Family first

Rosberg's childhood sweetheart, Viviane Sibold, and the new lady in his life, one-year-old Alaïa, will both appreciate having more time with the man of the house back in Monaco. Formula 1 remains a dangerous business. After two decades of good fortune without fatalities, the grid was reminded of this when Jules Bianchi crashed in Suzuka in 2014 and succumbed to his head injuries last year. The severe injuries sustained by Rosberg's former teammate Michael Schumacher, albeit not on the racetrack, might also have provided pause for thought.

Despite some uncompromising wheel-to-wheel battles with Hamilton this season, and most memorably their first-lap crash in Spain, Rosberg still displayed some signs that he was becoming a more risk-averse driver.

Consider the wet race around Monaco's perilous streets, where Rosberg could not keep pace with the leaders and settled for seventh as Hamilton claimed the victory. It was a stark contrast to the hard-charging youngster on show at Williams back in 2008 - the last wet and wild race at Monaco - when Rosberg scythed through the field for 59 furious laps in an uncompetitive car before smashing into the wall at speed at Piscine corner. 

Ending the Hamilton rivalry, still on reasonable terms

During their four years as F1 teammates, much has been made of Hamilton and Rosberg's single season together as juniors aspiring to reach F1. Hamilton alluded to it once again on Friday after Rosberg's announcement, celebrating the fact that both the teenagers' dreams of glory had since come true. 

In the past three seasons, battling each other for the world championship in a field where no other car could challenge the Mercedes, the pair's rapport has been repeatedly put to the test. All things given, it has just about survived the barrage - although the list of gripes and controversies is by no means short. 

Both drivers had 2017 contracts at Mercedes, but speculation that either could leave before very long has been rife for some time. By making way, Rosberg frees up Mercedes to give Hamilton the kind of uncontested senior status he would probably prefer within the team - and he also gives them the opportunity to look to the future, for instance by giving their young talent Pascal Wehrlein a shot in serious machinery.

Rosberg said how team boss Toto Wolff "understood" his decision, saying that abandoning his "racing family" was the hardest part of the move. Yet it is conceivable that Wolff was secretly glad to see Rosberg move on - defusing a potentially volatile partnership in the garage without Mercedes having to take action to keep the duo in check.

With 2017's sweeping rule changes, it's by no means guaranteed the team will be as dominant next season as in the previous three - and if the next title race is a close affair between teams, then Mercedes would not be able to afford to let two feuding teammates race without controls as Wolff generally has so far.

Underrated for years, now bowing out on top

Even as freshly-crowned champ, many still do not quite acknowledge Rosberg as a genuine member of F1's very top echelon. Former team boss Ross Brawn managed to damn Rosberg with faint praise around a month ago by saying that he'd love to see the German's "tenacity and application" be rewarded with a championship against Hamilton, the more talented but less diligent driver, in Brawn's experienced eyes. 

Rosberg, too, might be secretly thinking that beating Hamilton once is as good as things are likely to get. Fortune did favor the Wiesbaden-born, Monaco-raised son of Finnish 1982 F1 champ Keke Rosberg this season; not once did Rosberg retire from a race or a qualifying session because of mechanical failure.

This limited recognition has been a running theme of Rosberg's career. At first, many argued that his father Keke's money and connections helped explain Rosberg's rise to F1. When he then annihilated old Williams teammates like Kazuki Nakajima, people wrote off his direct rivals as "pay drivers," and therefore not a proper barometer. When he then beat returning champion Michael Schumacher in his early Mercedes years, many surmised that this showed how Schumi was now past it, rather than realizing the threat Rosberg posed even to F1's most successful racer.

Only up against Hamilton was the world finally made to take notice of the force Rosberg has become. Yet in four years, as Brawn said, Hamilton has generally enjoyed the slightest edge over his rival. By quitting now, right after turning those tables for the first time, Rosberg goes some way to ensuring the history books will always remember that he had the right stuff to win the championship.

Someone like Jenson Button, retiring this season aged 36, seven years after his lone F1 title - also secured at the Brackley-based team now called Mercedes, before Rosberg took his place in 2010 - might wonder whether he timed his exit quite as well.

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