A terrible week for consumer electronics firm Samsung got significantly worse on Tuesday as the US supreme court expressed limited patience for arguments defending patent infringements against Apple’s iPhone.

The hearing – which is the culmination of a five-year legal showdown between two of the world’s most powerful technology rivals – coincided with news that Samsung has had to abandon production of its new flagship Note 7 smartphone due to a safety crisis.

Meeting in the low-tech splendour of the highest court in the US, the eight justices seemed to prioritise protecting the design value of Apple’s iPhone, though the test case could have profound implications for intellectual property battles across the technology industry.

Samsung concedes the findings of the lower court that it copied the iPhone’s curvaceous case design and colourful graphical user interface for earlier models. But it argues that damages should be calculated as a relevant proportion of lost Apple profits, rather than the total $399m first awarded against it.

“A smartphone is smart because it contains hundreds of thousands of technologies that make it work,” Samsung’s lawyer Kathleen Sullivan told the court. The Korean giant argues only the profitability of the part in question should be assessed for damages.

Though the final ruling will not be known for months, the oral argument stage of the case made clear much of the bench was suspicious of the theory on the grounds that it would be almost impossible to ever determine.

“The problem is how to instruct the jury,” said Justice Anthony Kennedy. “If I was a juror, I wouldn’t know what to do.”

Instead, he suggested that courts apply common sense by throwing out vexatious claims over small items of infringement – such as a lawsuit against a car maker for all its profits in a case of identical cupholders – but allow companies like Apple to argue that exterior design can be a critical factor in its profitability.

“The phone could be seen by the consumer as just a rounded-edge outer shell,” Justice Sonia Sotomayor seemed to agree. “That might be what drives the sale.”

The ensuing discussion ranged from the original 19th-century statute used to determine the design of stoves and furnaces, to comparisons with other design icons where form was seen as just as valuable as function, such as the Volkswagen Beetle.

Justice Stephen Breyer described the issue of separating out the value of design from overall profitability as a matter “of great importance across industries” and warned that Samsung’s approach could lead to “absurd results”.

It was an hour of arcane discussion in court. The justices acknowledged the case was often confusing and could yet go either way, but the questioning of Samsung was noticeably more hostile than that of Apple.

“You seem to be arguing as a matter of law that you are right,” Justice Sonia Sotomayor told Sullivan at one point. “I don’t see that as a matter of law.”

Brian Fletcher, a lawyer appearing for the US government, took neither side and argued for a standardised test to decide future such cases. But he leaned in favour of the arguments made by Apple’s lawyer Seth Waxman that appearance was a vital factor in the company’s success.

“We know from Samsung’s own documents they realised they faced a crisis of design,” said Fletcher.

Even Samsung does not dispute the principle, but argues that it should be down to the plaintiff to prove damages. “It should be open to the patent holder to prove that the bulk of the profits come from the exterior,” said Sullivan.

In a statement, Apple said: “We firmly believe that strong design patent protection spurs creativity and innovation, ... And that’s why we’ve defended ourselves against those who steal our ideas. Eleven times now, Samsung has been found guilty of intentionally and blatantly copying the iPhone. Every court at every level has agreed. We think that’s wrong and that it poses chilling risks to the future of design innovation.”