AUSTIN, Texas (Reuters) - Dutch teenager Max Verstappen topped the timesheets as Red Bull lapped fastest in final practice for the U.S. Formula One Grand Prix on Saturday.
The 18-year-old produced a best effort of one minute 36.766 seconds, with Australian team mate Daniel Ricciardo 0.266 slower.
Ferrari's Kimi Raikkonen was third with triple world champion Lewis Hamilton fourth and title rival team mate and overall leader Nico Rosberg fifth for Mercedes.
Rosberg, who leads Hamilton by 33 points with four races remaining, and his team mate are the race favourites and sole championship contenders but Red Bull can play a big part in deciding the outcome.
Verstappen and Ricciardo are the only winners apart from the Mercedes pairing this season and have been getting closer to the dominant duo in recent weeks.
Hamilton needs to win all four races to reduce the gap to Rosberg but would welcome either or both of the Red Bulls taking points off his German team mate. Equally, they can help Rosberg by making life hard for the Briton.
The times will not have concerned Mercedes too much, with Red Bull using supersoft tyres to set the fastest times before Mercedes made the switch but were caught in traffic that prevented them showing their true pace.
More of a concern was the need for Mercedes to break a curfew on Friday night, with mechanics working late to make a precautionary change to the fuel system on Hamilton's car.
"On Lewis's car we are running now the engine we first ran in Spa," technical head Paddy Lowe told the BBC.
"From running it yesterday we found that there is a small fault in the fuel system, it is not a serious problem but for precautionary reasons we felt we should put a better one in there.
"The people working on the engine itself were in until 1 a.m. and then another group of people were in at four a.m. to do some more work and then the main group were in at six."
Editing by Tony Jimenez