Novak Djokovic has secured a place in the US Open semi-finals after fellow Serb Janko Tipsarevic retired through injury during the fourth set of their encounter.

The world number one was pushed all the way in the first set, but was ruthless in the tie-break, winning 7-2.

Tipsarevic won 7-3 the second set on another tie-break, making it the first set Djokovic has lost at this year's tournament.

While it was a close contest until that point, the number 20 seed began to experience a problem with his thigh and was swept aside 6-0 in the third.

Djokovic was leading 3-0 in the fourth before Tipsarevic was unable to continue.

The number one seed will face either Roger Federer or Jo-Wilfried Tsonga in the last four.