Del Potro ended Andy Murray's 14-match winning streak in the Davis Cup, coming through 6-4, 5-7, 6-7 (5), 6-3, 6-4 in 5 hours, 7 minutes to gain a measure of revenge after losing the Olympic final to Murray last month. It was the longest match of both players' careers.
Pella beat Kyle Edmund 6-7 (5), 6-4, 6-3, 6-2, meaning the Argentines can clinch victory with a day to spare in Glasgow if they win the doubles on Saturday. Federico Delbonis and Leonardo Mayer are scheduled to play the Murray brothers, Andy and Jamie.
Being taken the distance against Del Potro was exactly what Murray didn't need at the end of his long summer of tennis in which he won both the Wimbledon and Olympic titles for the second time. He looked almost out on his feet at times in an energy-sapping final set, and couldn't handle the barrage of forehand shots by the rejuvenated Del Potro.
"I'm very proud of how I fought, I did fantastic," Murray said. "I fought for every point, tried as best as I could. That's all you can do.
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"It was very fine margins. That happens in tennis and sport sometimes. It could have gone either way." Del Potro gained the crucial break in the fifth set at the third opportunity, reaching Murray's angled volley with a forehand winner down the line to go 4-3 ahead, and he served out for the match, clinching victory with an ace and a big growl.
Del Potro, the 2009 US Open champion, is a force in tennis once again after returning from three wrist operations that nearly ruined his career. He is Argentina's top player but is ranked a lowly No. 64 on his comeback trail, meaning he had to play Murray in the opening match.
"I cannot imagine this moment in my career," Del Potro said. "This was the type of match I was missing when I was at home."
Argentina is playing its 11th Davis Cup semifinal in 15 years. The south American country has never won the team event, finishing runner-up in 1981, 2006, 2008, and 2011.