VIENNA, Austria -- Mercedes motorsport boss Toto Wolff has praised the bravery of Nico Rosberg after the newly-crowned world champion announced his retirement from Formula One on Friday.

Just five days after sealing his first world title in Abu Dhabi last weekend, Rosberg made the surprise announcement at a press conference ahead of the FIA prize giving gala in Vienna. The news leaves Mercedes with a huge decision as it prepares to replace the 31-year-old over the winter, but despite the headache it creates, Wolff -- who found out on Tuesday -- said he could not help but be impressed by the Rosberg's decisiveness.

"This is a brave decision by Nico and testament to the strength of his character," Wolff said. "We've known each other for four years now and one thing I admire with Nico is his clarity of thinking. It's black and white.

"Although you could have argued after three days of celebrating you need to become sober in order to take a decision! But that is not with Nico, he knows exactly what he's doing, he's in control of his thinking, so from the first moment he told me there was no doubt that this is the decision."

Rosberg admitted he did not want to tell Wolff the news face to face as they returned from a series of press briefings in Malaysia, so instead waited until they were apart and informed his boss over the phone.

"I knew there was something as we spent a long time on the plane having dinner and talking about a lot of things," Wolff said. "The moment I left the lounge in the morning to jump on the plane to Stuggart, Nico was going to Weisbaden, he called me. I said 'I'm coming back', he said 'don't come back'!"

Wolff said there was no doubt Rosberg is bowing out as a deserving world champion.

"The result shows he won the world championship and he is the best driver in Formula One today. The points show that. The way I have perceived the last couple of races is that he was incredibly professional and did what was needed. That incredible winning streak at the beginning of the season built the foundations, and he just drove it home."

The Mercedes boss said Rosberg leaves big shoes to fill on several levels.

"He's left a huge legacy, first of all the way he's decided to leave Formula One is something that's unheard of, something that all of us admire in a certain way, to call it a day when you're at the top. He's been part of the development since the early days and such a strong competitor. The team has performed because the balance is right between the drivers and the dynamics within the team, travelling to 20 grands prix a year, the energy is hugely important and Nico was a big contributor to that energy in the team. I could go on for ten minutes...

"Besides the know-how, his skill as a driver, setting up the car, giving us direction on the set-up and the development of the car, direction on how to put the car on the race track, the interaction with Lewis and the engineers."