Red Bull head Mercedes in United States GP practice
Red Bull's Max Verstappen and Daniel Ricciardo were faster than the Mercedes in final practice at the United States Grand Prix.
Neither Mercedes driver completed a final lap on the fastest super-soft tyres and Lewis Hamilton and Nico Rosberg ended up fourth and fifth.
Championship leader Rosberg aborted his final lap while Hamilton was sent out late and got held up by a Sauber.
Hamilton told his team over the radio: "Really poor timing, guys."
It means Hamilton has not had a chance to simulate a qualifying lap ahead of one of the most crucial sessions of his season as he heads into the race this weekend 33 points adrift of Rosberg in the championship with 100 points still available in the remaining four races.
Hamilton really needs to beat Rosberg to keep his title hopes realistically alive this weekend.
The world champion's engineering crew sent him out for his final run at the very last minute, but as he neared the end of his warm-up lap he came up behind Sauber's Felipe Nasr.
Hamilton backed off to give himself some room to start the lap on the limit and only just made it across the line before the chequered flag fell. Even so, he caught the Brazilian halfway around the lap and had to back off.
That left Verstappen quickest by 0.266 seconds from Ricciardo and Ferrari's Kimi Raikkonen in third place 0.518secs adrift.
The impression - and that is all it can be after such a disjointed session - is that Red Bull are quick enough to put Mercedes under pressure when qualifying starts at 13:00 local time (19:00 BST).
However, Mercedes have the ability to turn their engine up further for qualifying and have usually had enough pace to stretch their legs and take pole position.
Ferrari's Sebastian Vettel was sixth quickest, ahead of Force India's Nico Hulkenberg, Williams' Valtteri Bottas and the McLarens of Jenson Button and Fernando Alonso.