Roger Federer conceded he was not in the right shape to challenge for another grand slam title as the elusive US Open clash with Rafael Nadal slipped away again.
After the Spaniard defeated Andrey Rublev, it seemed virtually nailed on that New York would witness a first meeting between the great beasts of men's tennis in the semi-finals.
Juan Martin del Potro, sick and exhausted after his epic comeback to beat Dominic Thiem on Monday, surely would not have it in him to beat the Australian Open and Wimbledon champion.
But the Argentinian again showed what a huge heart he possesses, saving four set points in the third set tie-break and going on to win 7-5 3-6 7-6 (10/8) 6-4.
This is the sixth time Federer and Nadal have come within one match of meeting in New York but the missed opportunity was not uppermost in Federer's mind.
The 36-year-old insisted it was his physical struggles, along with the form of Del Potro, that decided the match.
Federer arrived in New York nursing a back problem but appeared to have put the doubts behind him after surviving two five-set matches in the opening two rounds.
Asked if he was thinking about Nadal, the third seed said: "I wasn't. You were. It's not more frustrating. It's been a tough tournament throughout.
"I honestly was only thinking about tonight. My head didn't even wander during the match. I knew it was going to be a tough one. I had struggled too much throughout the tournament to think too far ahead.
"In some ways I'm actually happy I made the quarters, so I'm not disappointed, because it's been a good run this year already. Unfortunately I ran into a guy who was better than me today.
"Juan Martin deserves it more. I feel I have no place in the semis and he will have a better chance to beat Rafa.
"The way I played or am playing right now, it's not good enough in my opinion to win this tournament. It's better I'm out and somebody else gets a chance to do better than me."
Del Potro was the first opponent Federer had faced this tournament who had ever beaten him, and eight years on the Swiss had not forgotten the grand slam title that got away.
Federer led Del Potro by two sets to one looking for his sixth straight title at Flushing Meadows in 2009 only for the Argentinian to fight back and lift his first grand slam trophy.
At that stage it seemed inconceivable it would remain his only one but he began to struggle with wrist problems the following season and, four operations and two comebacks later, the search for his old game goes on.
After his epic comeback from two sets down against Thiem, saving two match points and fighting off the effects of a heavy cold, this was a remarkable effort.
Del Potro, who will play his first grand slam semi-final since Wimbledon in 2013, said: "I think I played my best match of the tournament.
"I did everything well, I served so good, I hit my forehand as hard as I can. I think I played a great match and I deserved to win in the end.
"I cannot believe to play another semi after all my injuries, all my surgeries. And especially to play here in New York. It's my favourite tournament, my favourite city to play tennis."
Del Potro's backhand may not be the same as it was after two operations on his left wrist that he feared would cost him his career but his forehand remains one of the game's great weapons.
Federer felt the full force of it as Del Potro gained the crucial break for 6-5 in the opening set.
The 19-time grand slam champion hit back as his opponent's energy began to drain away but then gave Del Potro a boost with a very poor start to the third.
Federer certainly should have won the tie-break, and how different the outcome might have been had he done so, but Del Potro took his chance and did not look back.
A defining feature of the match was Federer's uncharacteristically muddled thinking, and he explained: "I don't want to say I was in a negative mindset, but I knew going in that I'm not in a safe place. I might have depended too much on my opponent, and I don't like that feeling.
"I didn't have that feeling at Wimbledon or at the Australian Open, and that's why rightfully so I'm out of this tournament, because I wasn't good enough, in my mind, in my body, and in my game to overcome these three pillars.
"If you're missing all three, it's going to be tough. I'm okay with it, and I tried until the very end. Smashing certain stuff in the net that I normally wouldn't, smashing forehand volleys into the back fence, that stuff sucked."
The result also means Nadal will stay world number one. It would have been on the line had they clashed - another chance missed in the battle that might have been, and probably now never will be.