New York: World No. 1 Novak Djokovic entered the semi final at US Open on Wednesday after France’s Jo-Wilfried Tsonga was forced to retire from the match due to a knee injury. 
Frenchman Tsonga never looked in form and was trailing by 3-6, 2-6 before walking off from the match. 
This was Djokovic’s third win in five matches by default. Djokovic’s second-round opponent, Jiri Vesely, could not play because of a forearm injury.
His third-round opponent, Mikhail Youzhny, retired after six games with a hamstring injury. 
The Serbian has played only two full matches in the tournament so far.