Djokovic battles past an injury-struck Tipsarevic

Top-seeded Novak Djokovic advanced to the U.S. Open semifinals when his opponent, Janko Tipsarevic, retired with an injured left hamstring while trailing in the fourth set on Thursday.

Djokovic was ahead 7-6 (2), 6-7 (3), 6-0, 3-0 when the 20th-seeded Tipsarevic retired.

Djokovic had his own visit from the trainer after sliding to get to a drop shot early in the fourth set. The trainer came out and looked at his bleeding left toe.

Djokovic made it to his sixth straight Grand Slam semifinal and improved to 62-2 this season. On Saturday, he will play the winner of the later quarterfinal between Roger Federer and Jo-Wilfried Tsonga.

Even with Tipsarevic's early exit, the match took 3 hours, 27 minutes.