Nico Rosberg and Lewis Hamilton were involved in one of the most intense intra-team battles in F1 history when they raced for Mercedes between 2013 and 2016. After a season settling in together, they were soon fighting by themselves as Mercedes stole a march on the rest of the grid for the start of the turbo-hybrid era in 2014.
Hamilton, already a World Champion when he arrived at the team, would win the first two battles against Rosberg by notching up the 2014 and 2015 titles - navigating several flashpoints along the way. But in 2016 the fight reached a whole new level as they pushed each other to the limit, both physically and mentally.
At the end of a ferocious, then-record 21-race campaign, Rosberg came out on top by just five points, having clashed with Hamilton on the track and in the media, and been part of an overall dynamic that Mercedes boss Toto Wolff has admitted was "very, very difficult" to manage.
Such was the toll on Rosberg, he decided to retire from the sport just a few days later, recently revealing some of the sacrifices he made to achieve his dream. Since then, alongside F1 punditry roles, he has thrown himself into sustainability projects, aiming to make a tangible difference to the world of which he was previously crowned champion.
Rosberg's latest adventure has been to start up a team in the new-for-2021 off-road Extreme E series. An FIA-sanctioned championship, it sees identical 550-bhp electric SUVs go racing in some of the most remote parts of the world, with the aim of highlighting the climate change challenges faced by different ecosystems.
Hamilton was the first F1 star to confirm an entry, followed by Rosberg and fellow World Champion Jenson Button. Having raced each other week in, week out for several years, they are now on the other side of the fence, playing hands-on roles in the development of their respective teams.
"It's so funny that it's Rosberg against Hamilton!"
As it stands, Rosberg X Racing lead the way in the championship, just ahead of Team X44 (a reference to Hamilton's F1 car number). Hamilton's team have topped qualifying at all five events, though Rosberg's have won three races outright, giving them the edge in terms of points.
Extreme E's regulations require each team to field one female and one male driver in a move to eradicate preconceptions around gender divides in motorsport. Rosberg's team is represented by World Rallycross Champion Johan Kristoffersson and Australian Rally Champion Molly Taylor, while Hamilton is fielding World Rally Championship legend Sebastien Loeb and Dakar Rally regular Cristina Gutierrez.
Speaking exclusively to RacingNews365.com at the Bovington Camp military base in Dorset, UK, the venue for this weekend's championship finale, Rosberg says: "I'm excited! It's been a great season; we've won three races. Johan and Molly have done a great job, and all the team have in avoiding mistakes, problems - they've done really well. It's extreme conditions always out there.
"It's all to play for this weekend. It's so funny that it's Rosberg against Hamilton as well in this completely new championship here - that's amazing. Both of us have awesome drivers, so it's super cool."
As for whether they chat about their involvement in the series, and engage in any banter, Rosberg adds: "We did a little bit actually. I'm really happy to see how closely he follows it. I know he watches all the races live, he speaks to his drivers, he even writes to [series founder] Alejandro Agag, [for] some advice and thoughts and things - he's really into it."
Hamilton has yet to make a visit to the Extreme E paddock, though as Rosberg points out: "I think he would be [here] as well if he had time. Of course, I'm retired, so I have a lot more time than he does! But there's rumours that he might even come to take a look [this weekend]."
Hamilton following the action from a distance
Hamilton's team are on pole position around the muddy Bovington route, having topped both Qualifying 1 and Qualifying 2 - with Rosberg's drivers second and Button's pairing sixth. Under the standard format these time trials are followed by several multi-car races; the first- and second-placed teams from semi-final 1 and semi-final 2, and the top finisher in the bonus 'Crazy Race', progress to the final.
Gutierrez and Loeb therefore find themselves in the best possible position for Sunday's action, when they will chase an elusive first race victory. And as Loeb explains, Hamilton has been with them every step of the way, even if his F1 commitments - and fierce battle with Max Verstappen for the 2021 crown - have prevented him visiting the paddock up to this point.
"Lewis didn't have time to come this year, but when you see the intensity of the championship he's doing [in F1], for sure we understand," Loeb tells RacingNews365.com, fresh from completing a clean sweep of fastest qualifying times for the season.
"But we have always contact with Lewis during the races - before, during or after. I think he's involved, he enjoys what we are doing, he's always following closely what we are doing, asking some questions. I know he's interested in it, and that's nice.
"We won qualifying, so it couldn't be better than that at the moment. We are still in the fight; we are doing our best. It's the fifth time of the season that we won the quali; hopefully this time we can do a bit better in the semi and the final."
Loeb, a nine-time World Rally Champion with 79 victories and the best part of 1,000 stage wins to his name, adds of his involvement in Team X44: "For me, I'm proud to drive for Lewis. He's one of the most amazing drivers in the world."
Extreme E's efforts to change the world
Bovington is the fifth and final venue on the 2021 calendar, which has been split into several climate-themed categories. Back in April, the Desert X-Prix (Saudi Arabia) kicked off the season, followed by the Ocean X-Prix (Senegal), the Arctic X-Prix (Greenland), the Island X-Prix (Sardinia) and finally the Jurassic X-Prix (Dorset).
Driving change at each venue is at the heart of Extreme E's mission, with special legacy projects organised to provide support relating to specific local needs. These include the championship getting involved in turtle conservation along the Red Sea in Saudi Arabia, while in Greenland there is a collaboration with UNICEF to empower young climate changemakers.
Discussing the impact that is being made around the world, Rosberg explains: "We have some charitable work that we're doing as a championship, and then some that we do as a team as well. This weekend we're working with Mary McCartney (daughter of Sir Paul McCartney), with an NGO called FareShare, distributing 30,000 meals to families who can't afford their own food at the moment, so we're really trying to contribute."
Rosberg is also noticing plenty of energy among Extreme E's fanbase. Although events are not open to spectators, to minimise the local impact, fans can follow the action on TV and across social media, where Rosberg has been flooded with messages.
"The positive feedback from fans has impressed me the most," he says. "Everybody is so excited about it and following it, being like, 'Wow, I'm really engaged'. It's been really nice to see that welcome, especially on social media.
"[Going forward] the racing needs to be even more exciting - that's an important basis of everything. Then with that [we will] develop the audience, the fan base, get more and more fans, you know, which will come naturally."
And what about being tempted to come out of retirement to get behind the wheel once more? "No, I'm quite happy for them to do it!" Rosberg laughs in response. "They'll [Kristoffersson and Taylor] do a better job than me!"
All eyes are now on Sunday’s action in Dorset, as former F1 championship rivals Rosberg and Hamilton face off for title glory once again. But this time, what happens in the cockpit will be out of their hands…
You can follow the Extreme E finale from 10:00 GMT via the championship's official YouTube channel.