Lewis Hamilton is under investigation by the Mexico City Grand Prix stewards for a minor infraction during first practice in which Sergio Perez suffered an ignominious start to his home race.
Eighteen minutes into the opening hour-long session at the Autodromo Hermanos Rodriguez, Hamilton found himself unable to slow his Mercedes approaching the first corner on an extremely dusty circuit that caused numerous initial problems.
Hamilton, though, cut his way across the grassy run-off between turns one and two, rather than following race director Michael Masi's notes that make clear any driver taking such an excursion should rejoin the track by steering to the left of a bollard at turn three.
The seven-time F1 champion is likely to only be handed a reprimand but it serves as a warning, with any repeat across the remainder of the weekend leading to a penalty.
At the end of the session, Hamilton finished second quickest behind team-mate Valtteri Bottas, with championship rival Max Verstappen in his Red Bull a close third.
Bottas topped the timesheet with a lap of one minute 18.341secs on the soft tyres, with Hamilton 0.076s adrift and Verstappen 0.047s further back.
The early suggestion is Mercedes is at least in the game this weekend on a track that has previously favoured Red Bull.
But if you think back two weeks ago to the United States Grand Prix, Hamilton was nine-tenths of a second faster than Verstappen after FP1, only for Mercedes to find itself on the back foot to Red Bull beyond that.
Perhaps underlining Honda's likely strength at this circuit, Perez was fourth quickest, 0.269s behind Bottas, followed by AlphaTauri's Pierre Gasly who was just over six-tenths of a second slower than Bottas.
At on stage, it appeared as if Perez would miss the majority of the session after a crash emerging out of the stadium complex just 10 minutes in.
Just two minutes earlier, Ferrari's Charles Leclerc had slid into a barrier at the same point, causing slight damage to his rear wing and requiring a change.
Perez, with high hopes of becoming the first Mexican to win on home soil after back-to-back podium finishes in the last two races, made a similar mistake approaching the same corner.
Perez's RB16B bizarrely shifted one way under braking before sliding the opposite way, spinning around and clouting the same section of barrier as Leclerc but with significantly more force.
As Perez pulled away, he left behind half of his rear wing and after pulling back into his pit box, the suggestion at that time was that he may not return given the damage.
But the Red Bull pit crew again performed wonders that allowed Perez to return to the track and finish with a best time of 1:18.610s.
Behind Gasly, Ferrari duo Carlos Sainz and Leclerc were sixth and eighth quickest but with both over a second off the pace, split by Alpine's Fernando Alonso.
Esteban Ocon also fared well to sit inside the top 10 along with Aston Martin's Sebastian Vettel, with the four-time champion 1.5s down.
Team-mate Lance Stroll will start at the back of the grid this weekend, along with AlphaTauri's Yuki Tsunoda, due to the latest power unit changes.
The duo were 13th and 11th fastest respectively, finishing either side of Alfa Romeo's Kimi Raikkonen, who has been summoned to the stewards for the same infringement as Hamilton.
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