Title rivals Max Verstappen and Lewis Hamilton clashed in second practice at the US Grand Prix as Red Bull's Sergio Perez emerged with the fastest time.
Verstappen called Hamilton a "stupid idiot" over the team radio after an incident in which the two raced side by side down the pit straight.
Verstappen then gave Hamilton a middle-finger salute as they accelerated out of the corner.
Verstappen was told to "ignore it" by his race engineer Giampiero Lambiase.
Hamilton was third, his best lap deleted, while Verstappen was eighth.
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Hamilton set the fastest time of the session but had it ruled out for marginally exceeding track limits at Turn 19, which actually cost him time rather than gained it. The time he set was 0.1secs faster than Perez's quickest.
And Verstappen, who leads Hamilton by six points in the title race with six races to go, did not set a representative time at all and ended up eighth in a session in which he also uttered cross words at Lambiase after a mix-up over traffic, and then aborted a second attempt at a flying lap.
The incident between Hamilton and Verstappen happened when Hamilton passed the Red Bull going into the final corner.
The two then raced side by side down the pit straight, almost touching wheels, and into the first corner, before Hamilton claimed track position.
Verstappen said: "We all line up for a fast lap. So I don't quite understand what happened there."
The television cameras caught only the very end of a situation that had begun earlier in the lap when Verstappen passed Hamilton.
The Red Bull driver then slowed down later in the lap and Hamilton decided that he did not want to stay behind because he was trying to keep his tyres at the right temperature.
Verstappen initially appeared slower than Hamilton on the high-fuel race-simulation runs at the end of the session but on closer analysis the four cars from the top teams were evenly matched.
Verstappen said: "Overall it wasn't easy today as the track is quite bumpy. We've got some work on our hands overnight, but we will be back at it tomorrow."
Perez was fastest by 0.257secs from McLaren's Lando Norris, with Hamilton setting a fastest legitimate lap on tyres well past their best to be 0.364secs off the pace and 0.05secs quicker than team-mate Valtteri Bottas, who made a mistake on his fastest lap.
McLaren's Daniel Ricciardo was fifth fastest, ahead of Aston Martin's Lance Stroll, Ferrari's Charles Leclerc and Verstappen.
Bottas has a five-place grid penalty for using too many engine parts following an engine change.
Aston Martin's Sebastian Vettel and George Russell's Williams also have engine penalties and will start from the back of the grid, their penalties bigger than Bottas' because they have taken more parts.
Both Bottas and Perez had impressive days - they were the two quickest drivers on the long runs later in the second session, when drivers simulate the first stint of a race. Bottas was comfortably quicker than anyone else.
And the role of the number twos could be key in the remaining races as their teams battle for advantage in the championship.
Late in the session, there was an incident for Fernando Alonso, who spun his Alpine at Turn 19 and backed lightly into the barrier.
He seemed to get away with it without damaging his car.