Mercedes Formula 1 boss Toto Wolff says he is expecting Valtteri Bottas to take on a new engine ahead of qualifying for the Mexican Grand Prix.

Bottas pulled off to the side of the track in the stadium section of the Autodromo Hermanos Rodriquez in the closing stages of a damp FP3, with smoke pouring out of the back of his stationary Mercedes car.

Bottas reported he had received a warning on his steering wheel dash that signalled a hydraulic failure, prompting him to park his W09 in the final sector.

Asked about the issue immediately after the session, Wolff told Sky Sports F1: “The car is not back yet so we need to assess but it looks like a hydraulic failure.

“It’s probably going to be an engine swap. Let’s see if we can make it in time.”

Mercedes has since confirmed Bottas will avoid a grid drop penalty due to not adding a new engine to his pool of power unit elements.

The Finn had been running Mercedes' PU3 this weekend and will now be fitted with his PU4 engine, which he previously used in last weekend's United States Grand Prix. 

“The engine stopped and I got this warning [on the dash], I can’t say more,” Bottas explained.

“We need to investigate what was actually the issue. It was hydraulic we think, but we need to see.”

Lewis Hamilton, who can wrap up a fifth world title at this weekend’s race in Mexico City, recovered from Mercedes’ struggles on Friday to post the second-fastest time, 0.254s behind pacesetter Max Verstappen.

Despite the improvements, Wolff stressed his team is “not in a happy place” heading into qualifying.

“The drivers are not happy how the car is going, it’s sliding all over the place,” he said.

“We’ve clawed a bit back on the power unit but still there is a deficit so we are not in a happy place.”