Novak Djokovic cruised through to the quarter-finals of the US Open but Rafael Nadal suffered a shock defeat to Lucas Pouille on Sunday.
Djokovic needed treatment on his arm but was still too good for Briton Kyle Edmund with a straight-sets victory.
Meanwhile, Nadal was surprisingly knocked out of the tournament by Frenchman Pouille, who sealed victory after a dramatic five-setter.
Also through in New York are the French pair of Jo-Wilfried Tsonga and Gael Monfils, who beat Jack Sock and Marcos Baghdatis respectively.
DJOKOVIC FINALLY FACES A TEST
Having won through to the last 16 as the beneficiary of a walkover and retirement in the second and third rounds respectively, Djokovic was finally tested in the fourth.
However, despite battling an arm injury once again, Djokovic prevailed 6-2 6-1 6-4 at Arthur Ashe Stadium.
The defending champion cruised through the first two sets and had a slight hiccup in the third following his treatment, but still came through in comfortable fashion.
Djokovic will now face Tsonga - who got up 6-3 6-3 6-7 (7-9) 6-2 over Sock - for a spot in the semi-finals as he looks to win a 13th grand-slam title.
In one of the biggest surprises of the tournament so far, Nadal was unable to reach the quarter-finals at a grand slam once again - his last last-eight appearance coming at the 2015 French Open.
He was forced into a battle with Pouille who, despite having played consecutive five-setters, was able to come out on top in a 6-1 2-6 6-4 3-6 7-6 (8-6) marathon.
Nadal struggled for consistency in the early stages, never fully able to reach the decent form he has shown recently, but went down with fight.
Pouille will now face his compatriot Monfils for a spot in the semi-finals, after the latter cruised through against Baghdatis with a 6-3 6-2 6-3 victory.
With Pouille, Tsonga and Monfils into the quarter-finals at the US Open, at least one will make it into the semis.
With their fourth-round triumphs, it means there are three Frenchmen in the last eight of a grand slam for the first time since 1947.
NADAL STAYING POSITIVE, POUILLE CONFIDENT
"You can see it or you can write the way that you want. I know what's going on. I know what I have to do. It's just a couple of things that for some reasons didn't happen, and you need to be ready to have the auto-critic. You need to be auto-critic," - Nadal knows what he has to do to get back to his best.
"I think because mentally I'm stronger, physically I'm stronger, [that] gave me a lot of confidence before the match. I knew if I wanted to win that, it's not going to be like three sets, 6-1, 6-2, 6-2. It would be long. So I was ready for it," - Pouille was up for another five-setter.
"This particular situation I [have] never had in my grand slam career. It feels great to play a match," - said Djokovic, having only spent just over three hours on court before his fourth-round meeting with Edmund.
MURRAY, WAWRINKA EYE LAST EIGHT
Monday's action sees the fourth round wrap up, with Andy Murray and Stan Wawrinka among those in action. Murray takes on Grigor Dimitrov for a spot in the quarters, with Wawrinka facing Illya Marchenko. Also playing are Juan Martin del Potro against Dominic Thiem, and Kei Nishikori facing Ivo Karlovic.