The world number two came through 1-6 6-4 6-1 7-6 (7-0), but the scoreline barely tells half the story of a match in which Nadal fought like never before on his favourite surface, with Del Potro at numerous times in the match looking as if he could take it.
Del Potro was left in tears at the end after playing some of the most powerful tennis ever seen on a court. The crucial fourth set swung this way and that, but Nadal cruised through the tie-break to seal an emotional victory for Spain.
With the scores at 3-1, Spain retained the Davis Cup trophy, and under new ITF rules the final, dead rubber was cancelled to allow the coronation of the champions in front of an ecstatic home crowd.
Nadal started the first set strongly, breaking Del Potro’s serve immediately after the Argentine served up a double fault at break point. Del Potro replied in quite stunning fashion though as he reeled off six straight games to take the set.
The world number 11 hit 18 winners in that first set, 14 off his forehand wing, while Nadal managed three. It was as dominant a showing as has ever been seen against Nadal on clay.
Del Potro then won his seventh game in a row at the start of the second set, again breaking the Spaniard, who had not yet managed to hold serve.
Yet the following game witnessed a moment that ultimately turned the tie around, and with it the Davis Cup. Del Potro ran into a 40-0 lead, but Nadal somehow clawed his way back, courtesy of a string of errors off Del Potro’s racket. Deuce gave way to break point, and a final error put Nadal back level in the set.
From then on Nadal slowly turned the tide. The Argentine stayed with him until the latter stages of the set, though with every game the Spaniard was getting stronger. At 5-4, Del Potro serving, Nadal ripped a crosscourt passing shot and took the set when he smashed from close range.
Nadal did to Del Potro in the third set what the Argentine had done in the first, completely dominating from start to finish. He was lassoing winners off that muscular forehand at will, hitting more than twice as many as the Argentine to go into a two sets to one lead.
The final set was quite exceptional, despite starting out looking as if Del Potro was spent. He was playing conservatively, with a thigh that was by now heavily strapped.
Nadal broke immediately, racing into a 2-0 lead. It looked for all the world that it would drift to a comfortable win, but Del Potro started to find his range and broke back to level the scores, with Nadal now retreating on to the back foot.
But the Spaniard came immediately back to break again, cleverly moving Del Potro left and right as he struggled to get about the court. Yet once again, to the disbelief of the thousands packed into the stadium, Del Potro broke back, powering down inexplicable ground strokes into both corners of the court.
Two games later, the world number 11 broke once more to lead 5-3 after landing a defensive lob smack on the line and volleying a winner that looked certain to be sending the match into a fifth set. Yet that assessment was to underestimate Nadal, who pounced as Del Potro double faulted at a crucial moment when serving for the match and followed up with a passing shot at the third time of asking to break back.
Nadal then broke once again as the Argentinian hit an easy forehand into the net, and with Nadal serving for the match the Davis Cup looked over. Amazingly, however, Del Potro dug deep once more to break back with an extraordinary winner down the line to take the set to a tie break.
At that point, however, Del Potro's legs finally went and Nadal ran away with the breaker without dropping so much as a point, though by that time the match could already be filed away as a classic.
On match point Nadal rapped a forehand winner up the line, a fitting end to an amazing match. The Spaniard fell to the ground in celebration, but in a moment of class got straight back up to hug his inconsolable opponent.