Men’s quarterfinals Djokovic     bt     Del Potro     6-2, 7-6, 6-4Ferrer         bt    Tipsarevic     6-3, 6-7, 2-6, 6-3, 7-6NEW YORK - Defending champion Novak Djokovic reached the US Open semi-finals for the sixth consecutive year with a straight-sets win over Juan Martin del Potro.The number two seed beat the Argentine 6-2 7-6 (7-3) 6-4 in a match more even than the scoreline suggests.Djokovic, who has not lost a set in the tournament, was pushed hard before a second set lasting almost an hour and a half was settled on a tie-break. The Serb, 25, will meet Spain’s David Ferrer for a place in the final.Although not at his best against an opponent whose power caused him considerable problems, Djokovic’s superior athleticism and range of stroke eventually told.A forehand down the line produced the error that brought him victory in three hours and six minutes - an hour and 20 minutes longer than he had been held up in any of the previous four rounds."I love playing night sessions,” said Djokovic, who hit 43 winners to Del Potro’s 25. "We played some incredible rallies and some incredible points."I was lucky in the second set to come out with a two-set lead. Even though it was a straight-sets win, it was closer than the score indicated.”Djokovic will play Ferrer on Saturday, with the victor meeting the winner of the Andy Murray-Tomas Berdych clash in the final.