Defending champion Novak Djokovic reached the US Open semi-finals after Jo-Wilfried Tsonga became his third opponent unable to complete a match.
The Serb, 29, led 6-3 6-2 on Arthur Ashe Stadium when French ninth seed Tsonga retired with a left knee injury.
Djokovic has completed just two of five matches in New York, benefiting from a walkover and two retirements.
He will face another Frenchman in the last four after Gael Monfils beat compatriot Lucas Pouille 6-4 6-3 6-3.
Second seed Andy Murray takes on sixth seed Kei Nishikori on Wednesday, before Juan Martin del Potro and Stan Wawrinka play the last of the quarter-finals.