Glasgow - Juan Martin Del Potro shocked world number two Andy Murray in five sets to get Argentina off to a winning start in their Davis Cup World Group semi-final against holders Great Britain.
The 27-year-old, playing his first Davis Cup singles rubber in four years, sealed victory 6-4, 5-7, 6-7 (5-7), 6-3, 6-4 in just over five hours following an epic battle on the hard-court of the Emirates Arena in Glasgow on Friday.
The resurgent Del Potro had to produce some of his best tennis to defeat the Scot, who missed his paternal grandfather's funeral to play the rubber, and gain some revenge for his Olympic final defeat in Rio de Janeiro last month.
A win for compatriot Guido Pella, who faces Kyle Edmund in the second singles rubber, would leave Argentina needing just one more victory from either Saturday's doubles or the two remaining singles on Sunday to eliminate the holders and secure a fourth final spot in the past 10 years.
"It was an important victory for us," a delighted Del Potro said after a match that was, at five hours and seven minutes, the longest of both players' careers.
"I still remember the Olympics but this is important for me and the team and we are looking forward to reaching another final in the Davis Cup."
It was Murray's first home defeat in the competition and he thought there had been nothing to separate the pair in the rubber.
"It was very fine margins. That happens in tennis and sport sometimes when it could go either way and there wasn't much difference in the match," Murray said at the end of a draining day.
"I thought I did great today. I fought for every point and tried the best that I could and that's all that you can do."
Murray and Del Potro are the headline acts of the tie but faced each other on day one due to the Argentine's lowly ranking following his recovery from three wrist operations.
Three-time major winner Murray, roared on by a passionate home crowd, was playing his first match since suffering the disappointment of a quarter-final defeat to Kei Nishikori in the US Open.
At periods in the first set the Olympic gold medallist looked tired and flat as he produced a number of uncharacteristic unforced errors.
The crucial moment came in the seventh game as Del Potro broke the Murray serve to move ahead for the first time. The Argentine then produced a clinical display to seal the first set 6-4 in just under an hour.
The second was far better from Murray and the Scot made the breakthrough at the perfect moment as he finally broke Del Potro's serve to take it 7-5.
A moment of controversy turned the third set on its head. Murray, waiting on a let call that never came, stopped playing to allow Del Potro to seal the point that moved him 5-3 ahead. Fired up by this sense of injustice, the world number two produced some sublime tennis to win the next three games before taking the set on a tie break.
Murray looked uncomfortable as he moved about the court in the fourth set and an early break of serve allowed Del Potro to power to victory.
The Argentine showed little signs of fatigue in the fifth set as he broke the Murray serve in the seventh match before serving a remarkable 135mph ace to win the match.
Murray will have little time to recover and is expected to partner big brother Jamie in the doubles on Saturday.