Valtteri Bottas suffered a hydraulic failure during the latter stages of free practice three. The Finn's session was cut short as his W09 came to a half in the reformed baseball stadium section of the circuit.
Mercedes will have a huge task on their hands to identify the issue and rectify it in the short space of time between FP3 and qualifying, with the team not even receiving the car back in the garage until after the conclusion of the final practice session.
"The engine just stopped and I got this (hydraulic) warning," Bottas told Sky Sports. "We need to investigate the issue, we hope it's a small thing."
It will also cause a few nerves with teammate Lewis Hamilton's car, the Brit can wrap the world championship up this weekend if he can finish seventh or higher regardless of where title rival Sebastian Vettel finishes.
To compound issues, Toto Wolff also told Sky Sports that both drivers were complaining about the car's handling going into the all-important qualifying.
"The drivers are not happy with how the car is going sliding all over the place. There is still a deficit. We are not in a very good place.
"It's probably going to be an engine swap, we will see if we can make it in time."