The win is Djokovic's third of the tournament caused by an injury to an opponent
Novak Djokovic has reached the semi-finals of the US Open for the tenth year in a row.
The Serbian advanced after France's Jo-Wilfried Tsonga retired injured at the start of the third set, having lost the opening two. The win continues an unusual run for Djokovic in New York this year.
The defeat of Tsonga was third time in the past fortnight, that the 29-year-old won the match, without winning three sets. In the second round, he was given a walkover against Jiri Vesely, while in the following round. his opponent Mikhail Youzhny retired in the first set.
Djokovic will play Gael Monfils in the semi-finals. The French star defeated compatriot Lucas Poille in straight sets. It is the first time Monfils has reached the last-four of a Grand Slam, since the 2008 French Open.
Australian Open champion Angelique Kerber is through to the semi-finals for the first time since 2011, at Flushing Meadows. The German defeated Italy's Roberta Vinci in straight sets.
Kerber now meets Caroline Wozniacki. The Danish, former World number one defeated Anastasija Sevastova in just over an hour.
Wednesday night sees the men's and women's quarter-finals conclude. Andy Murray, Stan Wawrinka, Juan Martin del Potro, Serena Williams and Simona Halep are all in action.