AUSTIN (AFP) - Despite further engine issues, Mercedes dominated Friday's (Oct 22) opening practice for the United States Grand Prix with Valtteri Bottas edging world champion Lewis Hamilton and the rest of the field far back.
Bottas, who will face a five-place grid penalty after Mercedes changed his engine for the fifth time this season, recorded a lap of 1min 34.874sec with team-mate Hamilton just 0.45sec behind.
Red Bull's Max Verstappen, who leads Hamilton by six points in the title race, was a distant third, nine tenths of a second back.
There was another significant gap to Charles Leclerc, who was fourth in his Ferrari, 1.46sec slower than Bottas.
After Hamilton took an ultimately costly 10-place grid penalty for an engine change before the Turkish Grand Prix, Bottas and two other drivers with Mercedes power units, George Russell of Williams and Sebastian Vettel of Aston Martin, will incur penalties in Austin.
Drivers are allowed three engines a season, after that they are hit with decreasing penalties.
Bottas will drop five grid places for his sixth engine. Russell and Vettel will drop to the back of the grid after each switched to their fourth power unit of the season.
"Unfortunately, since mid-year we have had some reliability issues that keep coming back," Mercedes team principal Toto Wolff told UK Sky television, saying he was worried about cars not finishing the race (DNF).
"I think we understand now much better what it is but it means there is no durability in running them and that means a potential DNF risk," he said.
Hamilton and Mercedes are embroiled in a tight battle with Vertstappen and Red Bull for the drivers and constructors' championships.
Although Bottas won in Turkey, the Dutchman took the championship lead when Hamilton could only climb back to fifth.
"Penalties are not nice because it's such a tight battle," said Wolff.
He added that he was pleased that Mercedes dominated the opening session as temperatures rose into the 80s Fahrenheit on a track where Mercedes has taken pole every year since 2014.
"It's really good," he said, but noted that Red Bull have made a habit of gaining speed over the course of a race weekend.
"I don't know if it's gap that's going to maintain. I wish so but normally they are creeping their way back," Wolff said.
Behind Leclerc the drivers were tightly bunched.
The second Ferrari, driven by Carlos Sainz was fifth followed by Pierre Gasly in an AlphatTauri and then the second Red Bull of Sergio Perez.