• Formula One champion says he will leave the sport • ‘I have climbed my mountain, I am on the peak, so this feels right’

Nico Rosberg has stunned Formula One by announcing his retirement, just five days after the 31-year-old became the sport’s world champion.

He said he had “climbed my mountain”. Now he is going out at the peak. His Mercedes team-mate Lewis Hamilton, who won 10 races to Rosberg’s nine this year but was beaten by five points, said the move was not unexpected to him.

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The British driver said: “I’m probably one of the only people that it was not a surprise to. This is the first time he’s won a title in 18 years [though Rosberg was GP2 champion in 2005], hence why it was not a surprise that he decided to stop. He’s also got a family to focus on, and wants more children, and Formula One takes so much of your time. Will I miss the rivalry? Of course.”

Interest will now focus on the driver Mercedes will choose to replace the world champion. Fernando Alonso (McLaren), Sebastian Vettel (Ferrari) and Max Verstappen and Daniel Ricciardo (both Red Bull) would be obvious targets, but all are contracted to their respective teams for 2017. Instead, Mercedes could make a short-term fix and opt for another German, Pascal Wehrlein, 22, a member of the team’s young driver programme. Wehrlein has deputised for both Hamilton and Rosberg in testing.

The watertightness of Alonso’s contract at McLaren is the subject of some conjecture and F1’s chief executive, Bernie Ecclestone, says the Spaniard would be his choice. “It’s possible I suppose, let’s see,” he said. “They’ve got new management now at McLaren. Maybe he’s fed up being there and they’re fed up having him.” But a shocked Mercedes said they would not rush into making an appointment. Rosberg, who claimed his maiden title in the deciding race in Abu Dhabi on Sunday, made his sensational revelation at a press conference in Vienna, saying that the championship had always been his dream and, having clinched it, he would walk away. He signed a new contract with Mercedes for 2017 in the summer, worth £18.3m. Rosberg said he first started thinking about retiring when he won the Japanese Grand Prix in early October and realised the title was within his grasp. “From the moment when the destiny of the title was in my own hands, the big pressure started and I began to think about ending my racing career if I became world champion,” he wrote in a post on his Facebook page announcing his departure.

His mind was almost made up before his final, title-securing race on Sunday. He went into that race needing to finish in the top three and claimed that the process of considering his exit had cleared his head before the race.

Afterwards, he partied until the early hours before flying to Kuala Lumpur for a sponsors engagement on Monday. It was then that the German made his decision. “I didn’t know if I had the balls, and I took a bit of time,” he said.

The first people he told were his wife, Vivian, and manager, Georg Nolte, followed by the team’s head of motorsport, Toto Wolff. “Toto understood. He knew straight away that I was completely convinced and that reassured me. My proudest achievement in racing will always be to have won the world championship with this incredible team of people.”

He added on Facebook: “Since 25 years in racing, it has been my dream, my ‘one thing’ to become Formula One World Champion. Through the hard work, the pain, the sacrifices, this has been my target. And now I’ve made it. I have climbed my mountain, I am on the peak, so this feels right.”

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His battle with Hamilton dominated the season, after he had lost the 2014 and 2015 world title races to the Briton. Hamilton overcame engine problems and pushed Rosberg right up to the final race but could not overhaul the German, who admitted the fight had taken a toll on his family life.

“This season, I tell you, it was so damn tough,” said Rosberg. “I pushed like crazy in every area after the disappointments of the last two years; they fuelled my motivation to levels I had never experienced before. And of course that had an impact on the ones I love, too – it was a whole family effort of sacrifice, putting everything behind our target.”

Nico Rosberg (@nico_rosberg) I have a message for you: https://t.co/XtL5AWuY4n pic.twitter.com/LGK4Xnihoq

Wolff admitted that Rosberg’s decision was unexpected. “This is a brave decision and testament to his strength of character,” he said. “He has chosen to leave at the pinnacle of his career, as world champion, having achieved his childhood dream. The clarity of his judgement meant I accepted his decision straight away.” He added: “For the team, this is an unexpected situation but also an exciting one. We are going into a new era and there is a free Mercedes cockpit for the seasons ahead; we will take the necessary time to evaluate our options and then find the right path for our future.”

Hamilton has said he will not put pressure on Mercedes to pick a particular driver. “I have never been a driver to ever request,” Hamilton said. “I know a lot of the other drivers, like Sebastian and Fernando, make sure that is in their contract. I have just always asked to have equal rights. So as long as we are treated fairly then it doesn’t matter who is sitting alongside you. We have got great team bosses and I’m sure they will choose the right people to represent them.”