Nico Rosberg has admitted he took objection in Singapore when world champion Sebastian Vettel suggested his rivals are lazy.
After dominating the floodlit grand prix, German Vettel explained Red Bull's advantage by insisting that while his team works "very hard", the others are "hanging their balls in the pool very early on Fridays".
It may have been a joke, but Mercedes' Rosberg did not laugh.
"Maybe he shouldn't be thinking about my balls," he is quoted on Thursday by Germany's Auto Motor und Sport.
"It was definitely inappropriate," added Rosberg. "My team works its ass off. And he can't possibly know how much we work compared to them.
"My boys work like crazy."
And as F1 figures puzzle over why Vettel is being booed on post-race podiums, Rosberg said comments like the one made in Singapore do him no favours.
"He is in very clear danger of losing the respect of the other drivers," he said.
McLaren's Jenson Button agrees: "It was a wrong and unfair thing for Sebastian to say."
But Vettel said he found it "strange" that his comment was so controversial.
"It was not my intention to say anything against someone else," he said, "but only to show that our success is no accident."
Amid cheat claims, Vettel 'proud' of Red Bull 'system'
Oct.3 (GMM) There were two big talking points on Thursday as formula one gathered at the Yeongam circuit in South Korea.
The first was reports Sebastian Vettel has sped into a position of dominance in the second half of the season with the help of some sort of traction control-like system.
Red Bull and the FIA have dismissed suggestions it is illegal, and Vettel said on Thursday: "We are pretty proud of the system.
"The others have to figure out how we have done it," the German is quoted by the Associated Press.
The other big talking point is suggestions Davide Valsecchi has been briefed to be ready to step into the Lotus, should Kimi Raikkonen's back problems worsen in Friday practice.
The Finn was playing down the situation on Thursday.
"It would be a bit pointless to come here if I didn't think that I would race," he told reporters.
"Right now it's ok. Obviously tomorrow we'll see."
Red Bull to further 'enhance' traction system - Vettel
Red Bull is perfecting a "system" that is mimicking the effects of banned traction control, Germany's Auto Motor und Sport reported on Thursday.
The magazine said the complex electronic mapping system, although entirely legal, is able to put Renault's V8 engine down to just four-cylinders under certain circumstances.
"I am sure for the races to come that we will be able to enhance the system even further -- to profit even more," world champion Sebastian Vettel told F1's official website.
He told reporters: "Other people will never figure out how we've done it."
Mercedes' Lewis Hamilton on Thursday stirred the pot by admitting Vettel's traction in Singapore was not normal.
"The last time I was able to put the pedal down that quick was back in 2007 or 2008, when we had traction control," he said.
But Ferrari's Fernando Alonso admitted Red Bull, whose engine he said has been emitting the odd tones in corners all season long, is doing nothing wrong.
"They are using something different compared to the other teams but something that is completely ok," he is quoted by the BBC.
Red Bull boss Christian Horner, however, admitted the rumours were frustrating.
"So we're running traction control through the ECUs which are supplied by McLaren and approved by the FIA?" he bristled at reporters.
"You'd have to be pretty stupid," added Horner.