While the American lost to Andy Murray in Scotland, putting Great Britain into the quarterfinals of the prestigious teams event, halfway across the world an absorbing battle was playing out between two of South America's most bitter rivals.
In the end it didn't come close to matching Isner's 11-hour marathon against Nicolas Mahut at Wimbledon in 2010, but the fourth rubber in the clash between Argentina and Brazil made its own piece of history.
For six hour and 43 minutes, Leonardo Mayer and Joao Souza contested the second-longest singles match in tennis history -- and third longest of any format after a seven-hour doubles clash between Switzerland and the Czech Republic in 2013.
Mayer eventually triumphed, after his 11th match point, winning 7-6 (7-4) 7-6 (7-5) 5-7 5-7 15-13 in front of a delirious home crowd in Buenos Aires to send the tie to a deciding rubber.
It set the scene for Federico Delbonis to take on Brazil's Thomaz Bellucci in the decider -- with the winner to earn a clash with Novak Djokovic's Serbia, a 5-0 victor over Balkan rival Croatia.
Delbonis had won the first set 6-3 when play was halted for the day due to bad light. The match will resume on Monday.
Meanwhile, back in Glasgow, the British team celebrated after earning a home quarterfinal clash with France on July 17-19, the weekend after the completion of Wimbledon.
"It was an incredible experience," said world No. 5 Murray, who beat Isner 7-6 (7-4) 6-3 7-6 (7-4) in two and a half hours to give his team a 3-1 lead and ensure a repeat of the result of last year's first-round clash between the two nations.
"Playing in front of a home crowd is a bit more pressured, especially today, but I managed to fight my way through the first set and played really well after that."
It was the second defeat of the tie for the big-serving Isner, who lost a five-hour five-setter against James Ward on Friday.
The Swiss, without leading players Roger Federer and Stanislas Wawrinka, crashed out at the first hurdle with a 3-2 defeat in Belgium.
Canada makes their way into the quarterfinals for the second time in three years after beating Japan 3-2. Vasek Pospisil led the tie-break victory by defeating Go Soeda in straight sets on Sunday. Canada will face Belgium in July, after Wimbledon concludes.
France, beaten by Switzerland in last year's final, went through after beating Germany 3-2 -- losing both of Sunday's meaningless singles rubbers having already secured victory in the doubles 24 hours earlier.
Australia conquered three-time champion Czech Republic, a semifinalist last year, with Bernard Tomic clinching victory in the fourth rubber against Lukas Rosol.
Seeking a 29th title, but first since 2003, the Australians will next host Kazakhstan -- which defeated Italy 3-2 as Aleksandr Nedovyesov overcame Fabio Fognini in a five-set decider after trailing 2-1.