Valtteri Bottas lapped fastest for Mercedes in floodlit free practice on Friday as Formula One prepared to race in Qatar for the first time with the title battle entering its decisive phase.

AlphaTauri's Pierre Gasly was second in both practices with Formula One leader Max Verstappen, quickest in the morning but third in the second session.

Lewis Hamilton, who trails Verstappen by 14 points with three races remaining, was fourth in both sessions.

That was further disappointment for the defending champion. Stewards had earlier rebuffed the attempt by his Mercedes team to reopen the inquiry into an incident between him and Verstappen at last weekend's Brazilian grand prix in which Hamilton was run off the course as he attempted a pass for the lead.

Verstappen could potentially have been hit with a five-second time penalty, which would have dropped him to third and cost him three points in the drivers' championshp.

The decision added fuel to the enmity between the two team bosses.

Red Bull team principal Christian Horner and Mercedes Toto Wolff traded barbs as they sat side-by-side during a tense 30-minute media briefing that displayed the full animosity between them.

Horner said "there is no relationship" between the two and suggested the Mercedes team might be cracking as Red Bull attempts to end its reign.

"I think it's the first time they've been challenged. It's interesting to see how people react under pressure, how they react when they are challenged," Horner said. "It's by far the most intense political title fight we've been involved in in this sport."

Horner said he appreciates what Mercedes has accomplished "but I don't need to go to dinner with Toto. I don't need to kiss his (butt) or anything like that. There's a few other team principles that might."

Of the decision not to reopen the inquiry Wolff said: "What started as Olympic boxing went to pro boxing and is now MMA. Elbows are allowed now because the rules say so, and the gloves are off."

Horner added his team were ready to complain about the legality of the rear wing of the Mercedes claiming it flexed at speed.

"I think this is something even more advanced... it's hidden in the way that it operates so that it's harder to spot from a camera," he said.

"You can see the straight line performance since Hungary (in August), and particularly in the last two grands prix, has gone exponential. And I think that obviously concerns us."

Asked whether Red Bull were prepared to protest, Horner replied: "Make no bones about it, if we see it on the car here it will be protested."

Andrew Shovlin, Mercedes' head of trackside engineering, rejected the suspicion.

"From our point of view we're absolutely happy with what we've got on the car. We've invited the FIA to look at it as much as they want, they don't have any issue with what we've got," he added.

With Reuters