Red Bull's Max Verstappen led the field in the US Grand Prix's chilly opening practice session in Austin, the Dutchman leading Ferrari's Sebastian Vettel by 0.169s while teammate Alex Albon was third.
Toor Rosso's Pierre Gasly and Renault's Daniel Ricciardo completed the top five from which the Mercedes drivers were noticeably absent as both Lewis Hamilton and Valtteri Bottas appeared to spend much time in FP1 sampling Pirelli's 2020 compounds.
Teams were greeted with clear blue skies at the Circuit of the Americas, but temperatures were a cool 9 degrees Celsius, making FP1 colder than pre-season testing in Barcelona in February.
Lewis Hamilton hit the track with an aero rake fitted behind his Mercedes' rear tyres, an installation destined to gather data on Pirelli's 2020 tyres that teams were given an opportunity to test today.
But the championship leader radioed in almost immediately to complain about the track's surface.
"These bumps are insane out here - I don't know if this track is safe," were Hamilton's first words. as early footage indeed showed cars bouncing and sparking around COTA .
Bottas put the first fastest time on the board but the Finn was overhauled by Verstappen, with the Dutchman himself ceding the top spot to Haas' Kevin Magnussen.
The Dane's position at the top lasted a couple of minutes as teammate Romain Grosjean took over, only for Red Bull's Alex Albon to put everyone in their place with a 1m35.743s.
However, Albon's previous best time was deleted by the stewards for exceeding track limits at Turn 19, an infringement drivers will likely be mindful of this weekend, especially as Verstappen was also stripped of his best time.
Williams reserve Nicholas Latifi, running in his fifth FP1 session of the year in place of George Russell, was seen pulling off the track at Turn 3, the victim by the ominous sound of it of a gearbox issue. The Canadian was able to limp his way back to the pitlane however.
Meanwhile up front it was all about Red Bull as Verstappen and Albon - running on Pirelli's 2020-spec tyres - continued to trade fastest laps, with the latter lowering the benchmark to 1m35.282s.
But the bull's run was eventually cut short by a soft-shod Vettel who shaved almost a second off Albon's best.
Leclerc, who encountered an engine issue at the outset of FP1, was up and running once again, slotting his SF90 into P3 in between the two Red Bulls.
With 30 minutes remaining on the clock, the field appeared split between long runs and additional sampling of Pirelli's 2020 rubber.
But improvements were still in order as Verstappen reclaimed P1 from Vettel with a 1m34.057s, but the 22-year-old was again seen running wide at Turn 19.
While Albon moved up to third, Toro Rosso's Pierre Gasly, also running on the soft tyre, clocked in fourth, just a few tenths in front of Renault's Daniel Ricciardo, sporting his special 'Longhorns' helmet dedicated to Texas University's football team.
Oddly, neither Hamilton or Bottas were anywhere to be seen among the top ten, the Briton appearing to spend most of his session pounding around on the new-spec tyre, and continuing to complain about COTA's bumpy surface.
With five minutes to go, Mercedes finally showed a bit of its championship-winning hand, with Hamilton jumping to fourth. But low and behold, the reigning champ's time was deleted, once again for exceeding track limits! There won't be avoiding the ever-present timing loop at Turn 19 this weekend.
The session concluded with Verstappen, leading Vettel, Albon, Gasly and Ricciardo, while Grosjean was sixth, edging Leclerc, Hamilton, Racing Point's Lance Stroll and Magnussen who rounded off the top ten.