Daniil Medvedev, the only man ranked behind Novak Djokovic, has admitted that "some people from Serbia" messaged the Russian team during a dramatic day in which they ousted champions Spain to keep Djokovic's side in the Davis Cup.
The Russian Tennis Federation will return to Madrid to face Sweden in the quarterfinals after Medvedev and his teammates won first place in their group despite losing the first of their three matches against the holders.
In a stunning opening result, 40-year-old Feliciano Lopez, the world number 106, recovered from losing the first set 6-2 to beat Andrey Rublev, who is 101 places above him.
Known for thriving in front of a hostile crowd, Medvedev took on Olympic bronze medalist Pablo Carreno-Busta needing to level the scores to keep his team's hopes of winning alive.
The US Open winner surged into a 3-0 lead, then saw the first set out after Carreno-Busta broke back.
Carreno-Busta looked on course to force a deciding set when the world number 20 went 3-0 up in the second, only for Medvedev to win five games in a row.
The tenacious Spaniard broke back to ensure a tie-break which Medvedev won 7-3 to level the tie.
Djokovic had sealed victory for Serbia against Austria on Friday, but their previous defeat to Germany meant his team was relying on a Russian win to reach the final eight.
That looked in doubt when Lopez and Marcel Granollers took a first-set lead against Rublev and world number 18 Aslan Karatsev, only for the Russian pair to win 12 of the final 18 games to send their side through.
"I think we will not do this during the tournament," replied Medvedev when he was later asked whether Medvedev would buy him dinner in return for the result. "But another day - we'll see.
"It's funny, during the match we started receiving messages from some people from Serbia, let's call them that. But I won't tell you from whom.
"They said: 'come on, guys, we believe in you.' But our goal was just to win the match.
"It was not that we chose who to knock out, Spain or Serbia. We just wanted to win, and we are glad that we succeeded."
Karatsev called the Russian team "amazing" and "strong". "Of course, everyone comes here to win the title," he said. "Each of the players gives 100 percent. And yes, we are aiming for the title."
Spain captain Sergi Brugera also praised his players. "The truth is, I couldn't have been more proud of my team," he said.
"We have overcome a lot of troubles. We won 3-0 [against Ecuador] and then [only] lost 2-1 [against the Russian Tennis Federation], and [we won] the third set in doubles 6-4.
"My team fought incredibly. Today it's just great. I can't say anything bad, 10 points for dedication, for the level of play."
The Russian Tennis Federation will play Sweden and Serbia will meet Kazakhstan in Madrid on December 1 and 2.
Great Britain face Germany in Innsbruck on November 29, with Italy taking on Croatia in Turin on the same date.