Nico Rosberg has revealed that an ERS (Energy Recovery System) error was a key factor in his defeat by Mercedes team-mate and championship leader Lewis Hamilton in the US Grand Prix.
The German led the early stages of the race from pole position on Sunday, but he was unable to hold on when Hamilton passed him at the end of the back straight on lap 24.
Following a post-race analysis, Rosberg said he discovered that he had been left without extra ERS power to defend his position after requesting the boost via a switch change rather than using a button.
He said: "I made a mistake because I used the wrong thing to get more KERS. When I saw him coming I decided to go for extra boost, but I never got it.
"I thought I was using the right thing, but there was a delay in that one. If I did it with a button it would be immediate, but with a switch there is a delay, so I never got the extra KERS.
"I had a big drop off, which is why in the last metres he got a good jump on me. So, that was a mistake that I didn't know about."
Rosberg said he could have been more aggressive in defending the inside line against Hamilton, but added that it would have made little difference.