Mercedes lead both the drivers' and constructors' championships but team boss Toto Wolff admits they head to Japan scratching their heads over the inconsistency of their performances.

Lewis Hamilton finished second at the Malaysian Grand Prix on Sunday to open up a 34-point lead on main title rival Sebastian Vettel - who came home fourth having started from the back of the grid.

Max Verstappen took the race win having passed pole-sitter Hamilton at the start of lap four and then pulling away from the Brit with ease. Vettel, too, was quicker than his championship adversary for much of the race.

Hamilton had won in Singapore after Vettel, Verstappen and Kimi Raikkonen crashed out at the start and he was fortuitous here in Sepang when Vettel was unable to set a time in qualifying due to engine trouble.

Now the teams head to Japan with Wolff conceding all is not well at Mercedes despite leading the way in 2017.

"Full credit to Red Bull and Ferrari for their performance, but we have underperformed in Singapore and Kuala Lumpur - this is what we need to concentrate on," he said.

"I think we have built a points advantage to Ferrari in the last two races because of them shunting out in Singapore and Sebastian's engine problems here in qualifying.

"If they had performed as they should have we would not have increased our points advantage but lost many points to them.

"We need to understand why we underperform on certain circuits and in certain ambient conditions.

"I think we have a capricious car that has a very narrow operating window with the tyres. Dipping in and out of the window is the fundamental story of 2017 for us."

Wolff did also defend this season's Mercedes car, which has won nine of the 15 races of the year to date.

"We must not forget we are talking about the car that leads the constructors championship and leads the drivers' championship, it is not a lame duck," he added.

"This car has been on several occasions the quickest car out there, Monza and Silverstone come to mind where we blew everyone away."

Meanwhile Vettel insists he is confident of reeling in Hamilton after Ferrari proved they have the fastest car heading into the final five races.

Asked if he was optimistic ahead of Suzuka, he said: "Yes. I think we are quicker so obviously it was a bit of a shame. It has been a difficult weekend but nevertheless the speed is promising."

The second Ferrari of Raikkonen had qualified second but the Finn did not make the start due to a battery issue - although Vettel is unconcerned by the reliability.

"I hope not. I think we have it under control," he said when asked if reliability could be an issue.

"There are things we need to understand but this year we have a good record for reliability, I'm not too worried to be honest, we just have to get on top of these problems we have."