Mercedes Formula One boss Toto Wolff joked at Sunday's Abu Dhabi Grand Prix that he would be booking himself into rehab after a rollercoaster season that ended with his team winning both titles for the fifth year in a row.

If he did, the Austrian might find one of his drivers there already.

Valtteri Bottas started the year determined to fight, and beat, team mate Lewis Hamilton to the title but the Finnish driver ended it mentally drained and further away than ever from his goal.

"He said to me he just wants to disappear back home, recharge the battery, forget, recover and recuperate and then come back next year," Wolff told reporters. "I can understand the frustration."

Mercedes have hailed Bottas as a team player, a driver who does not play politics, and Hamilton has said they could be Formula One's best pairing ever. The statistics are less complimentary, however.

Bottas, who was testing again in Abu Dhabi on Tuesday, was alone among the top three teams' drivers in failing to win a race in 2018, the first winless in a Mercedes since Michael Schumacher in his final 2012 season.

Since the V6 turbo hybrid era began in 2014 Mercedes have always had both drivers finish the season in the top three -- until Bottas.

The Finn ended up fifth overall, a slide from third last year, and dismissed by some as no more than a useful wingman for Hamilton -- even if he did take the award for most fastest laps.

"It's been a mentally pretty tough year so I need a break," Bottas, who has no guarantee of staying in his coveted seat beyond next year, said on Sunday.

CAN DO BETTER

Some of it is down to bad luck, Hamilton's genius and the rise of Ferrari and Red Bull as challengers in the toughest season yet for Mercedes as the team to beat, but there were also painful mistakes.

"Confidence is not at the highest and he needs to try and pick it up again in the winter, because he can do better," said retired 2016 world champion Nico Rosberg, the man Bottas replaced, after the Finn finished fifth on track on Sunday.

"We've seen him do better and I hope he can find his way again."

Hamilton won more than twice as many races as anyone else this year but Mercedes had their least dominant showing of the last five seasons. They won 11 of 21, compared to 12 in 2017, 19 in 2016 and 16 in 2015 and 2014.

Ferrari won six, their biggest tally since 2008, while Red Bull's four was their most since 2013.

Hamilton made the difference -- equalling his best ever season's haul of 11 wins, for a career tally of 73, with 11 poles. He also became the first driver to score more than 400 points in a campaign and won the title with two races to spare.

The Briton signed off with a victory and intends to start next season in similar fashion, when he will be chasing a sixth world crown and closing in on Schumacher's all-time record of 91 wins.

The bad news for Bottas is that Mercedes have seen a change in their champion, which will not make the Finn's task any easier.

"He’s actually driven stronger after winning the driver’s title," Wolff said.

"I feel he’s just so embedded in the team and integrated that the driver’s title felt incomplete, which for a driver is really strange.

"When you look at his face, there was almost more relief and happiness about sealing the constructors’ title (than the drivers').

"That’s why he just didn’t take the foot off the throttle until the very end. That is somehow a new Lewis."