The McLarens fell off the pace in Canada on Friday afternoon, despite looking competitive in the initial practice session.
The buzzwords of the final 90-minutes of running for the day were extreme tire degradation and graining, with the Circuit Gilles Villeneuve's newly-resurfaced corners giving up precious little grip.
Many runners had to call off planned long runs on the softer 'option' tires because they lasted no more than a few laps.
"Everyone is struggling with the same thing, so I think it will be a pretty interesting race to see what happens with strategies," said Jenson Button, who was just 11th quickest in the afternoon.
His teammate Lewis Hamilton added: "I think a lot of people had the same experience, except perhaps the Red Bulls, so it's going to be an interesting one."
Indeed, having trailed the Mercedes engines and F-ducts in the morning, Sebastian Vettel set the afternoon's fastest time, with Ferrari's next-best Fernando Alonso also not complaining as loudly as the McLaren drivers.
"Friday's result is not that important, but it's looking not too bad," said Red Bull runner Vettel.
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