Novak Djokovic survived a brutal test from former champion Juan Martin del Potro to storm into the semi-finals of the U.S. Open on Thursday and stay on course to defend his title. The Serbian was at his brilliant best, producing 43 breathtaking winners and scrambling around the Arthur Ashe Stadium center court like a golden retriever, fetching everything the towering Del Potro threw at him. Djokovic eventually won 6-2 7-6 6-4 to go into the last four as the only man yet to drop a set in the tournament but the scoreline did not reflect the intensity of a match where he was pushed to the limit. Djokovic's next opponent is the Spanish baseliner David Ferrer, who won a four and a half hour slugfest with Djokovic's countryman Janko Tipsarevic. – Photos by AFP