* Valtteri Bottas joins Formula One champions Mercedes
* Finn replaces retired world champion Nico Rosberg
* Massa comes out of retirement to take Bottas's seat
* Brazilian veteran agrees one-year deal with Williams (Adds Bottas and Mercedes quotes)
By Alan Baldwin
LONDON, Jan 16 (Reuters) - Valtteri Bottas replaced retired Formula One world champion Nico Rosberg at Mercedes on Monday, with Brazilian veteran Felipe Massa coming out of retirement to take the Finn's seat at Williams.
Appearing at the factory with team boss Toto Wolff, Bottas promised to push new team mate and triple world champion Lewis Hamilton as hard as possible.
"I'm sure we are going to be close and both pushing each other forward," said the 27-year-old, who has yet to win a grand prix in four years at Williams but has appeared on the podium nine times.
Wolff, who helped guide the Finn's early career and also has close ties with Mercedes-powered Williams as a former shareholder, said Bottas would fit in well.
"He's very fast as a driver, and he shows our values," added the Austrian, who has had an unexpectedly busy winter following Rosberg's shock decision to quit 45 days ago, less than a week after winning his first championship.
"He's modest, humble and hard-working."
Williams had earlier announced that Massa, who left at the end of last season at the age of 35, had agreed a one-year contract to end the shortest of retirements.
Mercedes have been dominant for the last three years, winning both the drivers and constructors' titles, and will start as favourites again despite major aerodynamic rule changes.
MAIN MAN
While Hamilton, 32, can expect to be the team's main man - with many tipping the Briton to romp to a fourth title after winning 10 races in a rollercoaster 2016 - Bottas can be a genuine contender.
"Now it's time for the next level, to see how he can step up to challenge for race wins and for Championships," Wolff said.
"We know that we are already behind the curve in terms of preparations for the new season, so we've got a busy programme to get him integrated into the team. One thing is for sure: as I know Valtteri, he will give it everything."
Massa, a former race winner with Ferrari (NYSE:RACE) and 2008 world championship runner-up, will partner 18-year-old Canadian rookie Lance Stroll at Williams. always intended to race somewhere in 2017, but Williams is a team close to my heart and I have respect for everything it is trying to achieve," said the Brazilian, who made a tearful farewell to his home crowd last November. certainly have not lost any of my enthusiasm for racing and I'm extremely motivated to be coming back to drive the FW40."
Sauber earlier set the day's merry-go-round in motion by announcing that German driver Pascal Wehrlein, who was the Mercedes reserve and Bottas's main rival to replace Rosberg, would race for them this year. 22, drove for struggling Manor as a rookie last year but Mercedes felt they needed a more experienced replacement.
Bottas's arrival means Mercedes will have no German driver in their line-up for the first time since the German carmaker bought the former Brawn GP team at the end of 2009 and returned as a constructor.
Rosberg was partnered by seven-times world champion Michael Schumacher until Hamilton arrived in 2013.
Mercedes said Rosberg had accepted an ambassadorial role with the team for 2017, with his first commitment an appearance with Hamilton on Tuesday for watch sponsor IWC Schaffhausen.
The day's announcements left tail-enders Manor, who are in administration and seeking a rescuer, as the only team with any potential driver vacancies for the season starting in Australia on March 26.
(Editing by Ed Osmond)