Reigning Formula One world champion Nico Rosberg surprised everyone when he quit motor racing last year at just 31. He explained his motives at a Handelsblatt leadership conference.

(Source: dpa) Mercedes team driver Nico Rosberg celebrates winning the 2016 F1 Championship in Abu Dhabi.

On a cold, gray morning in November, Daimler CEO Dieter Zetsche got a call that left him speechless. Nico Rosberg, the recently crowned Formula One world champion for the Mercedes team, was quitting.

"I was at a loss for words, and not just because I had a toothbrush in my mouth," Mr. Zetsche recalled last weekend at the “Pathfinder” business leaders’ conference in Berlin, organized by Handelsblatt.

Mr. Zetsche's dismay was understandable. Mr. Rosberg was a sprightly 31-year-old and had just reached the peak of his career. He had celebrated the greatest triumph of his life after the final race of the 2016 season in Abu Dhabi, repeating a feat achieved by his Finnish father Keke in 1982. Mr. Rosberg was only the third German to claim the title, after Sebastian Vettel and Michael Schumacher.

For Daimler, which makes the Mercedes racing cars that carried Mr. Rosberg to victory, the young champion was just what the company had been looking for. Not only was he successful, he still had a long career ahead of him.

I had reached my dream. I can look back with full satisfaction. My career has fulfilled me. Nico Rosberg, F1 Champion 2016

Mr. Rosberg, however, had other plans.

At the Pathfinder conference, he and Mr. Zetsche shared a stage and discussed the Formula One champion's decision to bow out, the impact of that choice on Daimler and the reaction among fans and the media.

After Mr. Rosberg made his surprise announcement, some German media criticized him for abandoning his team of engineers and mechanics, who had also sacrificed plenty for his success. Mr. Rosberg's teammate and archrival, Lewis Hamilton, threw accusations of his own, calling Mr. Rosberg unsportsmanlike for not giving his opponents a chance to dethrone him.

But for all the scorn, there was an equal amount of admiration for a man who had climbed out of his cockpit for good in order to spend more time with his family.

"Since I was six years old, I have dreamed of becoming the Formula One World Champion," Mr. Rosberg said in Berlin. "Friends, family, leisure - everything else was secondary. The sport and success were paramount, day in and day out."

Mr. Rosberg said that wherever he went, his head was always at the race track. After crossing the finish line in one race, his mind would jump to the starting line of the next.

"Dedication is the will, the fighting spirit to beat everyone else," Mr. Rosberg said.

He remembered one race in Singapore, where the temperature inside the cockpit reached a scorching 55C (131F) and drivers were sweating three liters of water every two hours in their body suits. There are photographs of Mr. Rosberg after that race collapsed in front of a fan, exhausted.

Mr. Zetsche said it was that level of performance that separated the merely great from the very best. Ninety-nine percent of what race teams do is all alike - it's that last 1 percent that determines who wins a race, he said.

After following in his father’s footsteps, by 2006 the younger Mr. Rosberg had fought his way onto the Williams racing team. By 2010, he was hired by the newly founded Mercedes racing team, competing against giants like Michael Schumacher.

Nico Rosberg (left) and Daimler boss Dieter Zetsche speaking at the Pathfinder conference in Berlin.

After every race, Mr. Rosberg would go over his car piece by piece with his technicians. A good driver, he believed, should have a keen sense of how their chassis was tuned in order to ensure that it was optimally adjusted to their driving style.

Mr. Rosberg, his technicians knew, was a perfectionist when it came to details. He even had the stitches removed from his racing gloves in order to be able to shift faster.

Such dedication made it even harder for Mr. Zetsche to understand and appreciate Mr. Rosberg's decision, he admitted. “A part of my disbelief came from the fact that my own career had only started to really take off by my early 30s he said. “As time goes by, I'm beginning to understand his decision more and more."

Mr. Zetsche, who is now 64, said he still felt like he was at the "peak" of his career and didn't intend to step down as CEO until at least the end of 2019.

Six months have gone by since Mr. Rosberg announced his resignation and he still believes his decision was the correct one. “I had reached my dream. I can look back with full satisfaction. My career has fulfilled me," he said.

Mr. Rosberg will certainly have no money worries. He is said to have enjoyed an annual salary of €15 million ($16.5 million) at Mercedes and his total assets are estimated at €40 million.

But despite the financial security, he admitted he was still weighing what to do next. He doesn't have any concrete plans for his professional future yet, but nor is there much of a rush. He is only 31.

Markus Fasse specializes in aviation and automobile industry news and works from Handelsblatt's Munich office. To contact the author: [email protected]