Mercedes's two team bosses are refusing to celebrate the 2017 title yet.

With just four races to go, Lewis Hamilton now has a commanding 59-point lead in the world championship over Ferrari's Sebastian Vettel.

However, team boss Toto Wolff told Sport Bild: "Something is certain only when it cannot be another way.

"This year we have seen one of the toughest world championships in a long time, and now we have to keep doing everything to make the season a success for us."

Making Wolff particularly nervous is the unpredictable performance of Mercedes's 2017 car, and the sort of reliability problems that have struck Ferrari in recent races.

"Our car is still a diva," he said. "And Sebastian's fate in the last three races shows how fast things can change.

"In racing, the unpredictable can always happen and there are still 100 points in play. We will not take the foot off the gas until we have the title."

Mercedes team chairman Niki Lauda agrees, saying that while Hamilton is in a commanding position, the title is not won yet.

"Suzuka was an incredible step forward," he told Auto Motor und Sport. "But the title is still not in the bag.

"But it does look better and easier for us now."

The Formula 1 legend agrees with Wolff that despite being the favourite for the title now, Mercedes are still grappling to understand their 2017 car.

"At the moment, everyone is still searching for the answers," he said.

Lauda also admitted that the Mercedes is much better when leading a race, as opposed to when it is forced to chase a rival car.

"This is due to our aero package," said Lauda. "To avoid the problem, we need to be in front."

The season continues with the United States Grand Prix next weekend.