Top seed and defending champion Novak Djokovic was handed his third walkover of the US Open after his opponent Jo-Wilfried Tsonga retired injured at the start of the third set. The Serb will next meet Tsonga's French compatriot Gael Monfils for a place in the final.

It has been a strange Grand Slam for world No. 1 Djokovic, who has played only two full matches in the tournament now after being gifted by a walkover in the second round and a retirement after six games in the third round before his.

The former Australian Open finalist walked off to give the Serb passage into the last four for the 10th consecutive time after picking up an injury to his left knee when trailing 6-3 6-2.

Djokovic will play Monfils next after the flamboyant Frenchman outplayed compatriot Lucas Pouille in a comfortable straight-sets win.

Rafa Nadal's slayer Pouille made 44 unforced errors as 30-year-old Monfils dominated and has yet to drop a set in five matches at Flushing Meadows.

In the women's draw, Caroline Wozniacki will face top-rank chasing Angelique Kerber for a place in the final after both players won their respective quarter-finals.

Kerber proved too powerful for Italian Roberta Vinci 7-5 6-0 to reach the last four. The German is aiming to become world No. 1 if she equals or betters Serena Williams performance at the year's final Grand Slam.

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