Del Potro trailed 1-3 but at 5-5 he fired down a swinging ace to reach match point number five and this time Simon cracked, netting a backhand to the relief of the Argetine who roared his approval.
The third set produced more high-quality tennis and Nadal was the first who crumbled under pressure, losing serve at love at 4-5 to drop another set and face an elimination.
Nadal saved three break points before an imperious forehand victor put him 5-3 up and he put away a simple volley to finally finish off Del Potro, then made his way around the net to embrace his tired opponent as he lay on the court following another tumble. But he blew those three set points, the last with a double-fault he later called "a big mistake".
"I was close to beating him and I couldn´t because Rafa always has a little bit more than the rest of the players on tour".
Nadal started strong on serve to power his way into a 3-2 lead during the opening set. I tried to resist, maintaining focus.
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In the next set, Del Potro broke and served for the set at 5-4, but double-faulted and Nadal clinched the break-back with a forehand victor down the line for the second set to be decided by a tiebreaker.
Del Potro threw everything he could at Nadal in a pulsating match and even conjured the shot of the tournament as he acrobatically dived to his right to hit a fizzing forehand volley that ended a spellbinding 23-shot rally.
The world number one had lost only nine points on serve when, just before the ninth game of the second set, the result of Federer's shock demise was flashed up on the giant on-court scoreboard. At two sets to one down, the prospect of Nadal and Federer exiting a grand-slam tournament on the same day for the first time was becoming real.
But the second seed stayed alive by taking the fourth set before the gripping drama continued in an electrifying deciding set in which both players were left diving after volleys and slipping over as they sprinted after blinding winners.
Then Del Porto managed to hold serve to love, forcing Nadal to serve out the match, which he did, winning it 6-4 to reach his sixth semi-final.