Red Bull's strong chassis development this year, allied to good progress from engine supplier Renault, has helped lift the Milton Keynes-based team into a position where it is now able to threaten Mercedes.

And ahead of a major rule change coming for next year – with an emphasis on downforce that many expect to favour Red Bull – Mercedes motorsport boss Toto Wolff has no doubts about the significance of what is going on now.

"You just have to admit that they are doing a good job," said Wolff, when asked about the growing threat from Red Bull.

"They have closed up to us, and Renault is making a good job in catching up.

"The rules on the chassis haven't really changed in the last couple of years, so it is going to be more difficult for us in the last few races.

"But the truth is, the game for 2017 has already started...the bullet is out of the gun."

Although Red Bull's actual challenge in the United States Grand Prix was not as strong as Mercedes had feared after long run data from Friday, Wolff says that the fact the team is breathing down the world champion's necks was to be expected thanks to rules stability.

"We expected then to be a bit closer in qualifying and the race than it actually was, as on Friday they were very strong," he said.

"But it is what we always said – the longer you leave the rules alone the more the cars are going to converge on performance. So it is good we are changing the rules next year."