New Zealand's Brendon Hartley has posted the 13th fastest time in this morning's practice ahead of the Mexican Grand Prix.
The Le Mans 24 Hour winner was bothered by an engine problem all day and also spun in each session as he battled for rear grip. The engine issue saw him miss the majority of the first session while he was able to keep going in the second one.
He reported hearing strange noises from the engine on a number of occasions and eventually brought the car back to the pits with a little over five minutes remaining.
Despite the issues Hartley showed more talent in posting times faster than Lance Stroll, Stoffel Vandoorne and Kevin Magnussen as well as both Sauber drivers.
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Red Bull driver Daniel Ricciardo posted the best time - a 1:17.801s lap - which proved 0.121s faster than championship leader Lewis Hamilton. Max Verstappen was third with Ferrari's Sebastian Vettel fourth.
Hartley's Toro Rosso teammate Pierre Gasly also suffered an engine problem early in the second session but he was unable to take any further part in practice, eventually finishing with the 19th fastest time. It made it impossible to draw comparisons with Hartley.
They weren't the only team that had problems - Haas, McLaren and Red Bull also suffered problems at the high altitude Mexican track.
Earlier Hartley posted the 18th fastest time in the first practice session. The 27-year-old managed to complete just 10 laps after being forced into the pits with the issue.
Toro Rosso opted to run youngster Sean Gelael in the second car instead of Gasly in that first 90-minute session. Gelael completed 29 laps and set a time slightly faster than Hartley - good enough for 17th fastest.
Mercedes driver Valtteri Bottas topped the timesheets in the early practice, setting a lap 0.466s faster than teammate and championship leader Hamilton.
There is an additional practice session on Sunday morning before qualifying begins 7am NZT.