At the Davis Cup Finals in 2021, Daniil Medvedev has won all four of his singles matches.
The Russian Tennis Federation advanced to the Davis Cup final with a win over Germany.
The Russians took a 2-0 lead in the best-of-three semi-final after Rublev defeated Koepfer.
In the opener, Rublev needed just 49 minutes to win.
The US Open champion looked assured to spark Russian celebrations and tee up Sunday's final with Croatia.
Croatia booked their spot in the showpiece tie, which takes place in Madrid at 15.30 hours, with a 2-1 victory over Serbia in Friday's first semi-final.
The Russians were the pre-tournament favorites because they were banned from international competition for DOPING and have shown why with some strong performances.
'You should support me or I'll continue to win' - Medvedev baits Spanish fans.
He had won all three of his previous matches without dropping a set and he was never under pressure in this match.
The Spanish fans whistled while he celebrated victory, but he gestured to them to calm down and pointed to the court at the Madrid Arena.
The Russians eliminated Spain in the group stage, which led to a post- match interview by Medvedev in which he reminded the crowd.
The highlight of the two weeks has been the win over Spain, he said.
It was nice to beat the home favorites in Madrid and I'm happy about it.
He said that people don't understand how to make him lose. They should support me. I'm going to keep winning.
When he reached the US Open final, he fed off the crowd's negative energy.
After an even start where Struff had looked comfortable on serve, a sloppy game from the German helped Medvedev to break for 4-5 and he wrapped up the opening set with another strong hold.
It looked like the Russians wouldn't need a deciding doubles match to win.
After breaking for a second time in the second set and facing a break point, he did not disrupt his progress as the Russians reached their sixth Davis Cup final.
They have a chance to win the event for the first time in seven years.
Rublev responded with an imperious display against Koepfer in the opening rubber, despite doubts about his level after mixed fortunes at the Finals.
The world number five was focused throughout his win despite an early break in the first set.
