Roger Federer, Rafael Nadal and the ever-increasing possibility of a US Open meeting between the two were all overshadowed at Flushing Meadows by an extraordinary fightback from Juan Martin del Potro.
The 2009 champion has been struggling with illness and won only three games in the opening two sets against sixth seed Dominic Thiem.
It seemed doubtful whether Del Potro would even be able to complete the match but he did much more than that.
A dramatic end to a simply breathtaking match...
The Argentinian raced through the third set, saved two match points with aces in the fourth and then clinched the fifth to win 1-6 2-6 6-1 7-6 (7/1) 6-4.
At his moment of victory, after HawkEye showed that Thiem had served a double fault, Del Potro stood with head back and arms aloft as his legions of supporters on Grandstand roared.
On-court interviewer Pam Shriver had to wait for the 'Ole' chants to die down before posing the question of just how he was able to win the match.
"Oh my God," said the 22nd seed, who now must try to recover for a quarter-final clash with Federer in a rematch of their 2009 final.
Federer is used to being the main attraction but he could hear that attention was elsewhere as the noise from Grandstand drifted across to Arthur Ashe, where he was engaged in a routine 6-4 6-2 7-5 victory over Philipp Kohlschreiber.
Del Potro was one of the most popular players on tour even before his two long lay-offs with wrist problems, the most recent of which he feared would end his career.
It has been a very long road back, and Del Potro still cannot hit his backhand how he used to, but his heavyweight punch of a forehand remains one of the sport's great weapons.
He began to land blows on Thiem in the third set and was an early break up in the fourth before the Austrian moved to the brink of victory.
Thiem showed tremendous heart with thousands of fans screaming against him but his nerve let him down at the vital moment as, from 5-3 30-0, he allowed Del Potro to break back.
The 28-year-old clinched the tie-break with a forehand so powerful it took the break away, and in the fifth set simply refused to let his efforts be in vain.
Del Potro said: "I was trying to retire the match in the second set. Then I saw the crowd waiting for more tennis, waiting for my good forehands, good serves. I took all that energy to change in a good way and think about fighting and not retiring.
"I did well, and I started to enjoy a little bit more about the fans. I think I did everything well after the third set. The crowd enjoyed with me all the points. It was an unbelievable atmosphere."
Federer and Del Potro have not played at Flushing Meadows since the final eight years ago, when the Argentinian recovered from two sets to one down to win his only grand slam title.
Whether he will have the energy to do the same again must be very doubtful.
He said: "I feel tired. We cannot do magic. But hopefully my day off tomorrow I can recover a little bit more. Then I will see if I'm ready to play in good conditions against Roger."
Having been well below his best in five-set wins over Frances Tiafoe and Mikhail Youzhny in the first two rounds, Federer appeared to have shrugged off concerns over his form and fitness by easing past Feliciano Lopez.
There were no worries on the first score again here but Federer caused alarm for his fans by taking an off-court medical time-out at the end of the second set.
He explained afterwards it had been tightness in a muscle in his bottom, but added: "It's all good. No problems there. I'm not worried about it."
Federer is looking forward to locking horns again with Del Potro and trying to change the result from eight years ago, which ended his five-year winning run in New York.
"He was gone for so long that it's just really nice to see him back playing these kind of matches," said Federer. "That's what he came back for, to get crowd support the way he got it. We could even hear it on centre court. That's the first time I experienced that."
He added of 2009: "I felt like I left that match with a lot of regrets. It probably feels like one of those matches I would like to play over again. I feel like I would probably win it. I just had all these chances."
If Federer can win, and Nadal beat Andrey Rublev, the great rivals will face each other at the US Open for the first time.
Nadal played his best match of the tournament to beat Alexandr Dolgopolov 6-2 6-4 6-1 while 19-year-old Russian Rublev defeated an injured David Goffin to become the youngest men's quarter-finalist in New York since Andy Roddick in 2001.