Nadal suffered problems with his right knee during his third-round win over Russian Karen Khachanov but insisted it was not a major issue and subsequently won gruelling matches against Nikoloz Basilashvili and Dominic Thiem.
Nadal, who had never before retired from a US Open match, showed signs of distress nearly from the start as he limped noticeably after a gruelling 69-minute opening set. And while he still had the devastating forehand that's been one of tennis' signature shots and carried him to the 2009 title, repeated wrist injuries had made him one dimensional on the backhand side.
Nadal was a step slow in the second set, falling, 6-2, before retiring due to tendinitis in his right knee. Nadal was clearly hobbled, and the overpowering game of del Potro ultimately proved to be too much. In 2017, Nadal won his first Slams in three years (the French and U.S. Opens), then won his 11th French Open this year. "I hate to retire, but staying one more set out there, playing like this, would be too much for me".
The Wimbledon champion, who ended a two-year grand slam drought by winning at the All England Club, has now won 13 sets in a row in NY.
"Negative thing is two of them have been in quarter-finals and semi-finals of a Grand Slam, close to fight for titles".
I didn't take any pleasure in watching Nadal go through the motions in that second set - especially in case he did more damage.
Del Potro will face the victor of Friday's second semifinal between Kei Nishikori and Novak Djokovic on Sunday.
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He will face Juan Martin del Potro in Sunday's final.
With six years on the 37-year-old Swiss, speculation would no doubt mount as to whether 31-year-old Djokovic - who looks fit, healthy and motivated after a lengthy lay-off with an elbow injury - could even surpass his 20 major haul.
After edging a 69-minute set, Del Potro applied pressure early in the second, thundering winners off both wings to get to 15-40 in game two. "But at some point we had to take a decision".
One of the greatest players of all time, the one chink in the Spaniard's armour has always been his knees with injuries plaguing him throughout his career. "I love to play against Rafa because he's the biggest fighter in this sport".
But it was clearly galling for the player famed for his fighting spirit to hobble out of a second Grand Slam of 2018, having retired in the fifth set of his Australian Open quarter-final against Marin Cilic with a torn inner hip muscle.
Nadal, who reached the last four in NY on the back of brilliant shot-making and incredible movement, looked out of sorts early against Del Potro and struggled to cover the court with his usual tenacity. This is my favourite tournament, my biggest memories on a tennis court came on this court in 2009.
Many experts have long felt Nadal's physical style of play, which relies on power and athleticism in contrast to his more graceful rival Roger Federer, puts him more at risk of injury.
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