Red Bull's championship leader Max Verstappen called Lewis Hamilton a "stupid idiot" after the Formula One title rivals went wheel-to-wheel in US Grand Prix practice in Austin on Saturday (NZ time).
The 24-year-old Dutchman also gave the seven-time world champion, the most successful driver of all time, a single-digit salute as the Mercedes accelerated away on a hot Texas afternoon.
"Ignore it, don't worry about it," his race engineer told the youngster over the radio.
Verstappen is six points clear of Mercedes' Hamilton with six rounds remaining and the battle is getting more intense with every race.
Red Bull's Mexican Sergio Perez was quickest in the second session, with Verstappen frustrated by traffic and failing to set a proper flying lap.
He ended up eighth.
"Overall it wasn't easy out there today, the track is quite bumpy and to find the right compromise on the car hasn't been the easiest," says Verstappen.
McLaren's Lando Norris was second fastest and Hamilton third.
Mercedes had dominated the first session with Hamilton's Finnish teammate Valtteri Bottas fastest and 0.045 quicker than the reigning champion with Verstappen third, but nearly a second off the pace.
Bottas, fourth in the afternoon, will have a five-place grid penalty for Monday's race after an engine change.
Mercedes are favourites to win on Sunday after taking pole for the last six races there. Hamilton has won five of the eight since 2012, with last year's round cancelled due to the COVID-19 pandemic.
Ferrari's Charles Leclerc and Carlos Sainz were fourth and fifth in the morning.
The opening session was stopped after three minutes when Fernando Alonso's Alpine came to a halt on track. The team says the Spaniard, who returned later, had done so as a precaution.
Haas's Mick Schumacher and Perez clashed on the bumpy track late in the session, with the Mexican unimpressed by the German rookie.
F1 stewards decided to take no further action.