US Open: Novak Djokovic - Del Potro was tough

Defending champion Novak Djokovic says he's glad he got through a "tough match" after beating Juan Martin del Potro in straights sets to reach the US Open semi-finals.

The number two seed beat the Argentine 6-2 7-6 (7-3) 6-4 at Flushing Meadows meaning he is yet to drop a set during this year's tournament.

Djokovic will play Spain's David Ferrer in the last four, a player the Serbian describes as a "fighter".